The UFC held its first event in 1993 and is easily the most successful MMA promotion to this date. Many attribute their success to UFC president Dana White. He manages to popularize and grow the sport with every event put on by the organization. The UFC holds events worldwide, and is a major reason why MMA is the fastest growing sport of today.
A much smaller promotion, Strikeforce held its first event in 2006. The caliber fighters in this promotion don't compare to those of the UFC. However, Strikeforce provides healthy competition with the UFC, and is a good place for younger fighters to develop.
PrideFC was the biggest MMA promotion of its time. From 1997-2007, during a ten year run, Pride showcased over 60 MMA events, including a 2002 event that holds the record for largest MMA audience in history. Pride was eventually bought out by Zuffa in 2007 by the UFC. The Pride promotion was great for the development of MMA, and Zuffa still broadcasts "The Best of Pride Fighting" on Spike TV.