Is there a Yamaha software synth that contains the sweet, cool, live, natural, and mega voices yet? I really like the sounds in the Tyros, but I don't really need the whole keyboard--just the sounds.

No, and I wouldn't expect that anytime soon if ever since the MegaVoices especially are a big selling feature of both the Tyros and the Motif ES. Yamaha isn't real big on softsynths anyway - their own XG softsynth hasn't changed in quite a while.