The Freesound Project is a collaborative database of Creative Commons licensed sounds. Freesound focuses only on sound, not songs. This is what sets freesound apart from other splendid libraries like ccMixter.
Vocal & A-Capella samples - some of the singers request that you contact them first, if you plan on using their voices for commercial projects.
Free Sample Packs gives links to many different sample packs, mostly royalty free.
Soundbible contains royalty free noises and sound FXs.
Fruitytuts gives a lot of samples, instruments, soundfonts and other musical resources for free.
Hyperreal has a library of classic synth and drum synth sounds.
This article on Musicradar lists tens of links to sample packs with hundreds of sound each. All royalty free.