I second the endorsement of ESPN... I've made the switch from Yahoo, it's much better.
As for keepers, I have a couple recommendations based on my own experience. In my 12-team H2H keeper league, the rules allow each team to keep 4 players from their previous season's end-of-year roster who were taken AFTER the 4th round of the previous season's draft. Setting things up this way, I find, prevents the problems of a lot of keeper leagues where the really good teams keep their stud players and remain really good while the bad teams stay bad. For starters, it's only 4 players, so if you have, say, a 26 man roster, that's still 22 players that you're drafting. Also, it keeps most of the elite players (last year's Top 48) in the draft pool, so a lot of the big talent is recycled, keeping things more fluid.
Leagues with more extensive keepers settings can be fun but if most of the managers are having their first experience with a keeper league, I think you'll find doing things similar to the way I described to be more fun.