Another vote for Fantrax. We've been using it for 3 years for our keeper league and I have no plans to switch ever. The guys who run it are really responsive if you you have issues and care about improving the product. They've just gone through a major revamp of the site and are looking to develop a mobile app. For keeper leagues, it's really easy to roll over from one year to the next. They aren't perfect and we have had a few challenges but on the whole we're vert satisfied.
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Been using Yahoo for two of my keeper leagues and it's great. Extremely customizable (point systems, trading draft picks, can transfer teams from owner to owner etc.) plus they have a smart phone app. I'd recommend it.