Some of us have been playing this quite a bit for a few days, and it apparently does not have a topic, so:
Dominion is a fairly popular board game that also has at least one free [base game] online version, its a deck building game where you compete with 1-6 people to build a effective deck and get the most victory points. It has around 7.5 expansions, but even with just the base game the games got a fair amount of replay-ability as the cards you use to build your deck can be different everytime [unless you use prebuilts].
There are several hundred players online at any given time on this web version, so its fairly easy to just join a game. Further, only the host has to have the expansions to have the extra cards, and anyone can join even without them, so you can just join other peoples games who have the expansions, to experience/try them.
Also: really easy game to just play with some people in the DFC chat, as its free, and fairly quick to start. [Although it can take a bit to play through sometimes, and if one player quits/dc's the game ends.]
What the Game roughly looks like in the Html5 version:
Html5 version we've been playing:
http://www.goko.com/games/DominionWikipedia Entry:
http://en.wikipedia.org/wiki/Dominion_(card_game)Game Wiki with cards and stuff:
http://wiki.dominionstrategy.com/index.php/Main_Page