Since the freeware FX thread is doing well, how about we have one for instruments?
Razor already mentioned the huge collection available free for registering with Native Instruments, and I followed up with the free Kontakt drum instrument DrumMic'a.
If you're into combo organs, the first two I recommend are Combo V and Combo F, excellent modeled Vox and Farfisa organs. The developer went on to work for Arturia and expanded both of them into commercial products.
Dead Duck Software