As of 2015, free sound effects are available online at soundbible.com, freesoundeffects.com and freesound.org. All three websites offer royalty-free downloads, some of which require attribution and some that are in the public domain.
SoundBible.com updates its sound-effect library at least three times a week. Some of the free audio is only available for noncommercial use. However, royalty-free sounds that carry a Creative Commons attribution or are in the public domain are available for profitable projects as well.
Categories for audio effects offered by Free Sound Effects.com, include ambient, drum loops, weather, battle and animal sounds. The site also provides sound effects for professional use for a fee.
Freesound is a database of Creative Commons-licensed sound effects that users share and download. This website also features a blog and user forum.