I figured out a new Rain combo on the MK trilogy for PS2 that I've never seen posted anywhere. I was trying to see how many roundhouses you can get in there, I've gotten up to 5, but I think you can do an infinite number... it's just kinda hard to do. You can start the combo however you want, you just have to be closer to the opponent's side of the screen (if you're on P1 side, you need to be right of center, P2 side, left of center). I start it with the lightning, but you can use the ball attack or even just a roundhouse.

Lightning ->
(Run across) Roundhouse ->
Roundhouse (this one won't send the guy through the screen) ->
Lightning (before he hits the ground) ->
(Run across screen) Roundhouse (through the side of the screen)->
HP, HP, Roundhouse (through side of screen once again) ->
Uppercut (for an easy finish, 10 hit, 50%)

Instead of the Uppercut, though, if you're fast, after the last Roundhouse you can run a few steps away from the center of the screen (in the direction you just kicked, away from where the opponent will come flying in) and get two more punches and another roundhouse in there, but it's a real pain in the balls to pull off. But if you can get it right, you can keep doing it.