I'll second the Amplitube recommendation. I've used it on guitar, bass, drums, vocals, and lots of scary things.
I like a lot of the instruments on SampleTank, but I only have the version that came with ProTools. Ditto on Reason Adapted, which has a fabulous Wurlitzer sampler and some weird-ass synth stuff that's pretty handy on occasion.
I go back and forth on DVerb. There are settings I use for weirdo effects and occasionally straight vocals that work pretty well, but sometimes I do need to go outboard to get natural reverbs. As mentioned earlier, kinda depends on what you're looking for.