Sound FX.
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.
The Spanish Ministry of Education and Science offers a sound bank with...
"... sound resources to be used for educational purposes..."
These files are free in both senses (freedom and gratis). There's a directory organised in 16 categories (animals, communications, city, etc.), including around 6000 files. The website is in Spanish.