They always post fights from the main cards and some from the preliminary ones (if they include at least decently well-known names) not long after the actual events take place. Most of the post-fight interviews and entrances are cut, though. All you have to do is create an account--no paying or anything, and the videos' quality is good even though you may sporadically get fights with foreign commentary.
You can also watch boxing/kickboxing matches there. My only gripe with the site is the weird search engine.
Favorite fighters: Josť Aldo, Vitor Belfort, Carlos Condit, Benson Henderson, Lyoto Machida, Anthony Pettis, Georges St. Pierre, Junior dos Santos, Anderson Silva, and Erick Silva.