The Syntiac Pages, an information site about do-it-yourself synthesizer projects, offers SynFactory, an interactive, modular, analog-style, software synthesizer, for download. Loads of features, easy to use.
Coagula is an image synth, which means that it is both a program for creating and manipulating images, and a program for generating sound from those images. It creates some very unusual and interesting sounds. The Lite version is freeware for home users - Windows. Also, check out Granulab, a real-time generator of sound grains, created by the same author. Version 1.0 is freeware for home users - Windows.
At neXoft Software, download LoopAZoid, a feature-rich, 48-channel, virtual VST2 drum sampler with 64 stereo voices. This freeware for Mac and PC requires a VST2-compatible application and lots of computer power.
AudioMelody is a database of freeware and shareware audio tools, utilities and programs - all the software needed to create a virtual studio - reviewed and rated, with screenshots and up-to-date downloads.
AnalogX Vocoder allows you to use a WAV file of a vocal to modulate a WAV file of an instrument and make the instrument sound as if it's talking. Vocoder outputs 16-bit, 44100, mono WAV files, and it can load any bit-depth, any sample rate, mono WAV files.
DCOffset from AnalogX is a freeware DirectX Audio plugin which detects and corrects in real time the shifted-off-center waveforms that may appear in digital audio files, especially during A/D conversion, and that can cause pops and clicks, etc.