I have been looking trough the feeds, and I often see a post with someone that wants to give all of his dota keys away, but all of those posts are spread out, thus there is no main thread for that. And there shouldn't be one.
Still, I think a lot of people need to try dota, and may struggle to find keys, thus finding some on the forum. But only few people seem to have noticed the existance of so called dispensers. The logic behind them is: everyone who played dota for the last 2 years got about 20 keys to give to their friends. So many people have trouble giving them all away, while other search for keys because they have got no friend that plays dota.
I have linked a key dispenser right up there. The way it works is that you either give all of your dota keys to it, so that it may distribute them, or you demand a key from it. Details should be explained on the linked page.
I just want to get this out there, because I think with so many copies arround, nobody should have trouble finding keys, and nobody should sit on them till they aren't of use anymore.
I hope this helped, cheers :)