Alright, so let's start the forum off with one of my favourite T-shirt sites. If I had the spare cash, I would buy many a t-shirts from them. I talk about www.Unamee.com
The basic concept is that artists submit (or get commisioned or something, not sure exactly how it works) a Geeky t-shirt design, and it is available for purchase for 48 hours. After that, it's gone, never (possibly) to come back. I've been following the website for a while, bought a couple t-shirts as gifts, and I'm consistantly impressed by the webite. The shirts are fairly affordable too, $10 +shipping. Some of the favourite designs have been:
Now, the website isn't unique in the Limited Edition Geek T-shirts, but one of my favourite and if they keep going, I'll certainly be buying from them when I have some spare money.