Do you like any browser games? This is a special thread for them! I'm not talking about games from flash-game sites, I mean for example Travian.
Well in my view Hattrick is the best browser game ever! And it's completely free! I have been playing it since 2005. It was founded in Sweden and has nearly 1 000 000 active players at the moment. In spain, there's a record of 100 000 players!
It is an online football manager where you play with other managers. You can develop your team, youth team, players, .. in very different ways and I assure you after a couple of weeks, you will be addicted. I recommended this game to Traumize, he has taken the risk to play it and now.. He's addicted. ^^
This game takes you 10-20 minutes PER WEEK but you can spend hours on it every day. That's the beautiful thing about hattrick. You don't need to have hours of time. But you can spend hours on it. It also holds a forum with more then 100 000 posts a day.