Basketball, ping pong (table tennis), swimming, biking (sergio martinez), throwing football (marciano did often, thought it helped with overhand right), and especially wrestling. A lot of the old figters use to wrestle as part of their training regimen. Helps with stregnth, endurance, fighting in the clinches and on the inside, etc.
cycling, great for endurance, didnt Sergio do it.....Wrestling and swimming require endurance and use most muscles in your body wrestling i mean its a 1 on 1 sport, requires foot work, timing, technique, explosiveness, coordination, mental toughness, it is cut from the same cloth
Wildcard gym best gym Anderson silva learnt all MMA skill of ATG Roach [ame]http://www.youtube.com/watch?v=qntHWtZqWaY[/ame]
Basketball, futbol soccer, baseball, football. 2 boxing greats Marciano and Terry Norris were good minor league and high school standouts. James Toney whipped on Deion Sanders over a bed at a Michigan football camp. Any of the big 4 should help tremendously.
Pac wildcard training has sticks and stairs. ATG trainer Roach trains Pacman to counter counters with straight Right for southpaw.
Aside from other combat sports, soccer, ping pong, swimming, golf, and baseball all seem like they would help. Anything that tests your endurance is likely to be worthwhile too. The more macho sports I don't think have as big a correlation on performance as the above, though they may improve the mental side.