I like to play games online with Steam, but we often fall back into the whole 'What do you want to play?'.... 'I don't know, whatever you want to play' debacle.
Steam Roulette lets you add multiple Steam usernames and determine games that you have in common. It will show you both the top 5 multiplayer games you have in common as well as five random games.
To install, simply copy the files and update the config.php to use your Steam API key. You'll need PHP Curl for this to work.
The Steam Web API: http://steamcommunity.com/dev The un-published Storefront API: http://wiki.teamfortress.com/wiki/User:RJackson/StorefrontAPI