I think you should be more specific on what time of car you have or what you're interested.
There are many brand specific forums, as well as genre specific so we need more info to guide you here
you'll need to be more specific as to what "scene" you're looking for. for instance, in skiing you've got NS for park & TGR for more big mountain/bc type stuff. what kind of scene are you looking for in regards to automotive tuning? are you looking for the import scene? domestic big motor? etc.
ok disregard - you're faster than I am.
i don't really know of a good import tuner forum. i know honda-tech has a lot of good info, but it obviously honda specific. I don't know what else is available.