Is there a specific place to give out unused game keys?

You could try here:

http://www.giantbomb.com/steam/3015-718/forums/the-giant-bomb-steamgiftscom-giveaway-thread-551088/?page=49

Just don't post full keys in plain text, there are bots that can nab them. You can give them away anywhere you want, but please don't hold any complicated or lengthy contests or anything like that.