I also love Cakewalk's "Studio Instruments", which you can run standalone or as a VST plugin using vst~. It has a string section, electric bass, electric piano, and a drum kit. The sounds are good (with a suite of FX for each), and the interfaces look great... especially for the drums, which are as fun to watch as to listen to. With some patching magic going to the vst~ you can have them play all kinds of interesting stuff.
Inexpensive---I got my package for $30, though that was a sale at the time. Still, worth every penny.
Percolate gets a second vote from me; an example of a 'simpler' less expansive collection of models can be found on David Bessell's page - some very convincing wood, metal, string and wind instruments.