I know this is an old thread but just wanted to say it's been really useful for me. I'm looking for an old steel frame for a long distance bikepacking trip through some less-developed countries. I've been spending lots of time on Craigslist and visiting the local bike non-profits that take in donated bikes, fix them up, and sell them.
You guys have already created a great list in this thread, but also wanted to point out this list in case it's useful to anyone:
I just realized that I can't post a URL until I have 10 posts. If you go to a website called vintagesteelrider(dot)com and look for a page called "100 of the best vintage mountain bikes", you'll see the list I was trying to share.