The salary cap leagues I am in all use Yahoo.
We distribute and Excel sheet that has each team's roster and make each GM responsible for their own. Then every GM just e-mail their sheet to all other GMs. When trades occur you are expected to post your move and your updated salary.
There are severe punishments for exceeding the salary cap so, it is in everyone's best to properly manage your cap.
Sorry I do not have a site, we just use CapGeek far all our information.