Virtually all the sites offer play money games (I would say all but someone is bound to dredge up some site that spreads one real money table & call me a liar), so you can pretty much take your pick.
PokerStars is excellent, their customer support is absolutely top notch. Best in the business by most people's account.
I find Party to be annoying, though they are beta testing a new client that will make them very attractive for heavy multi-tablers.
Absolute is good for constant bonuses, but the games suck. Rocky tables, bonuses take a long time to clear at low levels.
Paradise is a decent site for really low limits (haven't played there since I left the nanos), the crowd there tends to be friendly.
Some sites seem to attract those who take poker seriously - Absolute, FullTilt, the Primas, & a few others are probably not a good place for beginners. PokerStars has somewhat of a reputation for tougher games, but I find myself playing there most often. I would also recommend avoiding the ex-Party skins if you're a newbie.
Viper GTS