Well, it really depends on what kind of car youre looking at. Most of the time, there is a specific forum for exact car you own, or are thinking of buying. I find that these are normally the best place for advice when it comes down to specific technical issues and general how to type information for a car, bar none.
For example, the NAISSO forums,
http://www.naisso.net/naisso/forum2002/default.asp , have more than enough information for 1994-1996 B-bodies, and some other cars, like the newer impala. however, i wouldnt want to hang around there for fun. its downright boring for general chat. its the type of place that you go to ask a question, and come back a couple of days later to see what everyone had to say.
and thats why i like it so much. no chance for threads to be derailed, or forum drama pushing your important technical issue off of the first page.
now, for quick and dirty answers, for problems that might arise as im halfway through a front end suspension swap, or something along those lines, i would go to a general help forum, like Automotive Insanity over at Something Awful.
http://forums.somethingawful.com/for...?s=&forumid=91
now SA is a pay website, but its well worth it. you can browse some of the other forums, but to get an idea of what im talking about, you really should check out AI and see if thats your cup of tea.