One of the hottest things to do on Steam is to make your own maps for the games you love. I'm not exactly sure of all the games you can make maps for, but I do know Left 4 Dead 1 and 2 are some of them.
Left 4 Dead maps: http://www.l4dmaps.com/

Me and my friend always download new maps from that site to add to the gaming experience. A lot of them are a lot of fun, even better than the original 8 maps L4D2 on Steam gives you.

We've tried making our own maps but it's so damn hard.
Steam also gives you the tools (mainly Hammer World Editor) to make your own maps too, which is awesome.