There are many sound FX libraries out there with cool files that let you decorate your music productions, be it with a background ambience or a poignant buzz. The usual rule of the thumb stating that price means quality has some truth in it; what you find floating around the Internet for grabs tends to be of poor quality... but not always.

SoundTransit is a collaborative, online community dedicated to field recording and phonography. You can plan a sonic journey through various locations recorded around the world, or you can search the database for specific sounds by member artists from many different places. If you are a phonographer, you can also contribute your recordings for others to enjoy.

Here's a collection of links to websites offering free sound effects. The quality of the listed websites may vary, but a bit of perseverance will let you find exactly what you're looking for —if relatively common, mind; "tentacled hermaphrodite alien chewing an Austrian astronaut" may be tricky to locate. There are also links to websites offering free loops and soundtracks.