For the third straight year Soniss have released a massive collection of sound effects for download in celebration of the annual Game Developers Conference. You can see the complete list of released audio files here, in total there are 806 audio files across dozens of different categories from several different creators. The license seems exceptionally liberal with very few conditions:
Restrictions
a) Licensee may not modify any of the sound effects with intent to claim authorship of the original recording.
b) Licensee may not sell any of the sound effects as they come. (Although the sound effects may be sold as incorporated into licensee project).
Rights Granted
a) Licensee may use the licensed sound effects on an unlimited number of projects for the entirety of their life time.
b) Licensee may use and modify the licensed sound effects for personal and commercial projects without attribution to the original creator.
c) Licensee may freely distribute the licensed sound effects and make an unlimited amount of copies.
d) Licensee may publicly perform a reproduction of the sound effects over any form of medium.
You can read the complete license here. Downloads are available from multiple sources including Dropbox, Google Drive and Torrent links available, click here for various download links.