I experimented with several online chess sites today, and my choice for best site (for the casual player) is MSN.
Unlike most of the other free sites which use Java applets, MSN uses ActiveX controls. In other words, it permanently installs the program onto your hard drive. So although it seems like you’re playing inside a web browser, it’s really a client/server application. The benefit is that it starts up a lot faster because you don’t have to re-download the Java applet every time you play.
The standard rated game at MSN is 10 minutes per side. I find this more civilized than the more common 5 minute blitz (you have twice as long to think), but you can still finish a game in 20 minutes at most.
MSN automatically pairs you up with someone close to your rating, unlike all the other sites where you have to find someone willing to play a game with you. The MSN system means you get started playing right away. The downside is that you don’t have the sense of community that some of the other chess sites offer, because you can’t see who’s playing and you can’t observe other players’ games.