Ok, we have some sort of society, more or less ideal we are trying to reach. Such as society without class or wealth gap, racism and sexism. Delegation minimized, and all delegates with imperative mandate and immediate recallability. Means of production in the hands of the workers, production organised ecologically sustainable way. And so on.
Aim of the political discussion is to proceed these ideas, get more people involved to make better society and so on. For political discussion, you often need some sort of shorthand notation for what do you stand for. The purpose is to propagate ideals through these labels, aim is to get some concept widely known. When concept is known, discussion can go further. But the problem is that all semantics is a matter of contract, but there is strong enemies which are against these contracts. They are against our attempts to define ourselves in the political discussion in the fields they are involved in, and they are quite succesfull in it since they are very strong.
So, market liberals will be against all our attempts to claim that we are anti-authoritarian and anti-capitalist, since it is in their interest to claim that all critical to "market", as they call capitalism in order to mislead, are totalitarians. Authoritarian communists will be against all our attempts to say we are against the authorities and against the capital, since it is their interest to claim that only alternative to the private capitalism is the state capitalism. Nazis will be against our attempts to say that we are anti-nazi, anti-capitalist and libertarian, since it is their interest to claim that only 3 choices exist.
Concepts like "left", "socialism" and "communism" have so many competiting meanings, that I think it is purely strategic question depenging on each different field of debate how to use these labels. In the game to change the society, the winning strategy can be to choose a label, and to defend some of its common meanings against the others. If those enemies, which try to maintain unequal society or to create a totalitarian one are trying to take over some label, you can try to show their dirty attempts. If enemies are trying to throw mud on some label you've chosen, you can try to clean it's image. Another strategy is to give labels to your enemies, and attack the labels by giving them the new meaning in accordance with their new users.
More wider the audience, more better chances our enemies have to make the rules they want. Of course it is a catch 22 situation if you want to call yourself a leftist or not. But saying that you are not something leaves always a bigger liberty to define yourself, if there is more choices than 2 around. And to claim there is only 2 choices is a game created by the authoritarian communists and the social democrats, and it has been extremely succesfull game for them. They managed to destroy anti-authoritarian left in most of the world by this game. I disagree with claim that "I've rarely seen people have much of a doubt about what constitutes the radical anti-authoritarian left", I would be surprised if more than 50000 people in this planet know what the heck that means. So people, think twice before you choose their rules!
What is this left then? I've read that during French revolution those who wanted quick reforms were sitting in the left, and those who wanted slower ones were sitting in the right in the people's convention, or whatever it was called. So right from the beginning you can ask where the anarchists are in the scale, since anarchists do not sit in the parliaments? 150 years after the French revolution the situation was completely different, those who wanted planned economy were sitting in the left, and those who wanted free market were sitting in the right. Again it is a question where are the anti-authoritarians, since they are against the both ideas? In the French revolution "righ-wing radical" would have been a controversy, nowadays everyone knows what it means. But there became also plenty of rightists for protection or other forms of controlled economy, and often the scale was that more you were against the authoritarian communists, more you were in the right. And since anarchists were most bloodiest enemies of the authoritarian communists, what was their place then?
Robin-Frans Winkel asked what do I mean if I am talking about "black spots" in the "original" leftism. I think black spot of Marx was the idea about proletarian dictatorship. Fascist system Lenin created around the idea of vanguard party is of course partly twisted interpretation of Marx, but a lot of it is just natural evolution of Marx's authoritarian ideas. I do not see so strong empirical evidence against Marx to exist since birth of a single totalitarian communist country resulted that the others became similar. But what it comes to the Social-democracy, empirical evidence against it is completely overhelming. There is not a single social-democratic party which has not become completely rotten. It was the 12 social-democrat goverments of 15 EU goverments who were for example behind the idea of moving MAI into WTO. USA, which has been controlled by extreme right during almost all of its indepence, is after NAFTA much less interested about trade liberalization than EU. Social democrats are our deadly enemies in the sense of the second degree mass murder they cause in the South, minuscule groups like new-age fascists or nazi-skinheads can be more mentally ill, but for sure not less evil. And it is not always just second degree, it was Mitterrands pals who cut 500000 people into pieces with machete knives back in the 1994.
Alain got it right when showing the bridge the concept of "progress" builds between the left and the right. If you read papers by (Finnish) social democrats, the solution is never nothing else except the trickle down. (Of course we should also know well, that since social democrats are more clever than market liberals, they are sometimes ready to put barriers the against capital accumulation if they find it useful. For example currently they see riding with nationalist and racist sentiments in accordance with their aims, and they do it by building barriers against the free immigration).
So left was rotten since its creation, there is no any historical, "pure" left to reclaim from the scum which nowadays marches under the name of leftism. The only real criteria wether to use the label "leftist" is if it is useful for your goals or not.
In the US, "leftist" can be useful label since the bastards present in the left never imposed their sick rule, wether social-democrat or leninist there. In the central-European countries, where strong anti-authorian left tradition exists, "leftist" can be useful label to maintain that tradition. But in Finland, since 1918 there was no other left except authoritarian communist and social democrat. If you pick the label, at first you have to explain that you are not authoritarian communist. And it is difficult, since authoritarian communist try to save themselves from extinction by copying all the ideas and rhetorics from the other radical currents. If people believe you, the question is wether it makes point to choose "left" current with about 1000 supporters, anti-authoritarian one, or "left" current with far more than million supporters, the social democrat one. Each 4 years social-democrats come to the streets with their "we care" propaganda, all "left means equality" or "left means humanity" rhetorics helps them to maintain their lies. And the same in ex-USSR, all the claim that some sort of left-right distinction exists here profits nothing else except the red nazis.
Also in the places like Italy where strong anti-authoritarian tradition exists, many former leftists ("Negrist" current) have dropped all the political isms and labels, feeling that this move gives them larger right to self-definition.
I use sometimes label "anarchist" to define myself. It is also problematic since anarchist tradition is full of really disgusting people (Netsayev, Ravachol, Unabomber) and people who deserve good beating if still lived even if they had many beautiful ideas (racist, sexist and chauvinist Proudhon, anti-semitic and panslavic Bakunin, Kropotkin who backed France in the WW1). But usually I only use anarchist label in "left" arena like this list to counter other leftists, in larger arenas I often abolish it. And in some arenas I might be leftist. But to tell what you really are, you always need more words than one meaningful phrase may consist.
Of course I do not define myself as "third way", it has ugly connotations and since private- and state capitalism are not ways at all, it is stupid to say that there is some third way.
And last, when I mentioned "right infiltrating left", of course I did not meant that social democrats were left anytime since 1914. I meant that posers looking for money and power favour social democrats to rightists nowadays, since their position is much stable if they are in the "left" - cheated people have no-one else to vote.