Free

Spitfire Audio LABS - Lots of high-quality instruments. My favourites include Drums, Bass Guitar, Soft Piano, Strings, and Peel Guitar
Surge - Free and open source hybrid synthesizer
Vital - Spectral warping wavetable synth
Keyzone Classic - Pianos
Ample Guitar M Lite - Acoustic Guitar (Download Page)
Ample Bass P Lite - Bass Guitar (Download Page)
Native Instruments/Kontakt Komplete Start - A wide range of instruments, samples, and effects
ReaPlugs - Compressors, delay, EQ, gate, and more by the creators of REAPER (Windows only)

Not Free (But Reasonably Priced)