Public Domain and Royalty-Free Sound Effects Clips
NASA Sound Effects
This is a collection of sounds suitable for use in programs or as ringtones for your phone. The collection includes both live sounds as well as voice announcements and communications.
http://www.nasa.gov/connect/sounds/index.html
Wikimedia Commons
https://commons.wikimedia.org/wiki/Category:Sound
Sound Bible
A good collection of sound effects. Some of these are in the public domain, others are licensed under the Creative Commons license. Make sure you check and comply with the terms.
PACDV Free Sound Effects
This is a nice collection of free effects.
FreeSound
A large collection of sounds with a high percentage of ambient and soundscape type sounds. Creative Commons licensing.