Who owned Monday night football?

Technically, the NFL owns the rights to every NFL game. It sells rights to broadcast those games to the highest bidder.

Pete Rozelle basically blackmailed ABC into signing a contract for Monday prime time games in 1970. ABC's Roone Arledge took the idea and ran with it, turning MNF to the second-longest prime time program in history. ABC aired 555 Monday night games before the game moved to ESPN in 2006. Currently, ESPN owns MNF rights through 2013.

Sources: Wikipedia, San Francisco Chronicle.