I really like Spelunky, probably to an unhealthy level. Anyway, it was just on offer as a flash sale in the Steam sale and I bought 5 copies to give away so others could enjoy it as well. All you have to do is post in this thread and tomorrow I'll randomly pick 5 winners. You have to have at least 500 forum posts or 1000 wiki points or had something featured in the User Community Showcase to enter.
I've also got a few other games in my Steam inventory (Cogs, Frozen Synapse, McPixel x2, Hitman: Blood Money and Hotline Miami) that have been lying around for a while that I might as well give out. You can pick one of those as well and I'll select a winner at random for that too.