There are sound effects libraries that cost more than a small car, and they’re probably worth it to certain kinds of users — like movie studios or audio production houses — but not to me. In search of interesting, appropriately licensed sounds for personal amusement, some google searching led me to Freesound.org, which has many thousands of freely usable, user-contributed sound recordings, all Creative Commons licensed. Some of them are tiny snippets, the audio equivalent of the icons on a computer screen, and some are lengthy field recordings. (Many of the sounds here are purely synthetic, too, or remixes that the CC licensing facilitates.) Last Halloween, I set up a playlist for my family’s “haunted condo,” consisting of screams, clanks, and creepy laughter (but also repurposed sounds like foghorns and musical instruments I thought sounded ominious), with sounds drawn entirely from this site.
It’s also a good place to find ring-tone and computer alert sources, if you’re just looking for audio clip art, or (with headphones, especially) fascinating “you are there” audio experiences; being transported to an audio landscape inhabited by gentle waves, ships’ horns, and thunderstorms is a legal way to escape ordinary consciousness.
Freesound really is free, too, though donations are accepted; it started as a project of the Music Technology Group of the Universitat Pompeu Fabra. One (very small) catch: you can listen all you want just by visiting the site; downloading the files requires free registration.