A group dedicated to getting gamers together to play various games or just hang out. Reddit Playdate also has a subreddit: http://www.reddit.com/r/Playdate/ and steam group: http://steamcommunity.com/groups/redditpd .
Playdate has two official playdates per week. One on Friday (8pm est) and one on Saturday (8pm est). The Friday game is decided by the group mods and the Saturday game is decided by the subreddit.
Some more information about Playdate can be found on the reddit wiki here: http://www.reddit.com/r/Playdate/wiki/index