Whether you're looking for crickets for the a little ambient pop in the background of your camping montage, or robotic squeaks and grinds for the main character in your space-based rock opera, The Freesound Project has sound effects for you.
The thousands of sounds at The Freesound Project are licensed under the Creative Commons Sampling Plus 1.0 License. You're free to use, with attribution, the sound effects in any non-commercial setting — songs, videos, etc.
Samples are user-organised with tags and related samples suggested based on what other users downloaded along with the files you're currently browsing. You can search files by keyword, file name, and perform more advanced searches via file bitrate, samplerate and more.
The Freesound Project is a free resource and requires a simple email registration to download files.