There is a daily trivia contest on the site http://nflplayers.com
I've actually won this twice but I'm not sure if there are winners monthly or daily (or weekly even). The prize(s) change monthly but are typically a box of football trading cards. One box I won was worth about $15-$20, the other $150.
There isn't really a rules page, after you submit answer, you see the prize, which is a little backwards. Sweeps is daily ongoing.
[If you're not sure of the answer, enter bad name/email info, find out the right answer, and then go back and enter again with the correct stuff.]