Here are some great free audio plugins/libraries, far more better than entry-level softwares (GarageBand, etc.) :
– Kontakt 4 player with included sounds. The player version only features the first 3 convolution reberb categories (17 in the full version), however they cover most needs and sound great.
– Kore player with included sounds, Compilation Vol.1 and Compilation Vol. 2
– Camel Audio Alchemy Player with provided sounds (very high quality)
– SampleTank Free with included sounds (very high quality)
– Yellow Tools Independance Free (most of the best sounds from the Basic version, including acoustic and electric basses and guitars, and the famous NDB organ with Origami free reverb).
– EastWest Symphonic Orchestra Free
– Sonatina Symphonic Orchestra
– Native Instruments Guitar Rig Player
All these softwares are really worth paying for the full versions, that include more sounds. Moreover they are fairly priced and can sometimes sound even better than some high priced libraries.