GamerZap

We have to give credit to Yahoo for providing a high-quality fantasy football system - despite the fact that the vast majority of players aren't paying a penny to play.  You'll never have a problem finding a league to play in here.  Every year Yahoo increases the strength of their system to better handle the growth in numbers of leagues and fantasy football players.

There are a couple of paid options that give you access to additional tools for managing your teams.  You can upgrade your league to the "Plus" level and everyone gets to use a decent batch of draft and stats tools.  You can also purchase access to the "Stat Tracker" that will give you a look at up-to-the-second results of your leagues.  It's a single small fee for the full season and easily worth it if you're the kind of fantasy player who wants to know where they stand after the first group of games on Sunday afternoon.

Public leagues put you in with a random group of other fantasy team owners.  Custom leagues let you invite your friends and tweak the system to include stats of your choice (we particularly like being able to use defensive players instead of full defensive squads).  Leagues can be set up to use a full-season points system or to go head-to-head between teams each week.

In addition to regular fantasy football, Yahoo also offers Survivor Football, Salary Cap Football, and both College and Pro Pick 'em games.  New in the past year is the "Fantasy Sports Profile" that keeps track of all your Yahoo results and even displays trophies for your top finishes!