Crash of the Titans is a really fun 3D platformer. It brings back a lot of the fun of the PS1 games. The only flaw I had was that it was WAY too short.
Tak Great Juju Challenge is a really pretty 3D platformer. If you can survive the horrible driving minigames, then you might find a good game. I did!
Over the Hedge surprised me. It's a movie game, but it has some surprisingly good platform and sneaking elements that are inspired by Metal Gear Solid. I don't know if you'll like it, but it's worth checking out.
Shame the DS is lacking in good 3D platform games. We know it can pull of decent 3D graphics and decent controls with the Dpad. The developers just don't push it very well. And the Rayman DS port could have been so great, but it was messed up with a terrible framerate and laggy controls. It wasn't the DS' fault since it's done much better in other games. It was just badly ported.
And Soma Bringer is not true 3D. You can move in all directions, but the backgrounds are 2D still images. The characters are the only thing in 3D.
I'll also mention one more game that technically isn't a platformer. It's Pac n Roll. A Pac Man game fully 3D. You use the stylus to roll Pac Man around nice levels. It's fun, so you might try it.
And then Frogger Helmet Chaos is 3D too. And it's definitely a platformer. But the style is an acquired taste. If you've played a Frogger game before, you'll know what to expect. If not, just know that the entire game is based around grid layouts and strategic jumps.