Can you give me an example of left wing thought that is worthy of respect even if it isn't current?
I can offer some possibilities: the anti-Jim Crow movement of the 50s and 60s -- but this was really started back in the 1800s by Republicans, so I consider it an ethical issue rather than liberal or conservative. Welfare -- but this goes back at least as far as jewish law, which mandated "leavings" for the poor at harvest time.
I would argue that most of the issues with which liberals justify their supposed moral superiority are neither liberal nor conservative. they are matters of personal character.
I'll take you literally - your mean thoughts rather than thinkers. Also, I have to be a little loose in my interpretations since ideas are cumulative - they build on the past.
How about the counter-syclic economic ideas of John Maynard Keynes? The kibbutzim of the Zionists? Fabian socialism and syndicalism? Modern trade unions? Universal health care? Social Security (even though that's usually attributed to a very conservative 19th century German)? Child labor laws? Environmental preservation? General tolerance (although this one too has many fathers)?
That's just off the top of my head. Also the anti-slavery laws which you attribute to Republicans - who were very left-wing by the standards of the time. Keep in mind also that Adam Smith and capitalism represented Left-wing liberals in his day. On the right were aristocrats, monarchists, and much different economic ideas.