As far as racing games are concerned, this is a sore spot for me. There are no four player split-screen racing games on the PS3 (or 360 for that matter). I'm a very social gamer and do most of my gaming with others. So the lack of a good four player racing game has been a sticking point for me. MotorStorm is an excellent racing game that would be great to enjoy with my friends, but its lack of splitscreen prevents the game from being played much. I hope MotorStorm 2 corrects this problem.
The closest I have is SEGA Rally Revo, a two player split-screen rally racing game. It's a lot of fun, but it's quite limited and I was hoping for more. For example, why does split-screen have a vertical split instead of horizontal? That makes sense for an FPS, but not a racing game! But it's the only PS3 split-screen racing game that I like.