MFL is great. But I will also throw in a plug for the free version of Yahoo. It is polished and allows you to give everything back as prize money.
We have used it for years for a friendly keeper league similar to the one you are trying to find a home for. As long as your scoring is fairly standard and you can handle a little setup for the online draft, it works well, looks nice, has great reports and player info, etc.
Yahoo does not support keepers out of the box. So to be safe, every team *must* place the keepers from all the other teams in their exclude list. Assuming that is done properly you should be good to go. Either draft the keepers in the first # of rounds or make sure each owner drafts them in the round they are being kept in.
One additional comment. If live scoring on Sunday is important to members in your league, the MFL is probably a better choice for you. The Yahoo league is free, the live game day scoring is not. By the time every member in the league pays for that, you have probably paid for MFL which includes its own live scorer. And the "Pro" version of Yahoo, while it does include the live scorer, is much more expensive than MFL and not as flexible.