Freesound is a good and well-known resource, although not every sound there is libre (some sounds are under non-commercial licenses*) and unfortunately you have to have an account in order to download anything. There still are good sounds there however, and a good amount of them are libre.
(*Only one non-commercial license, CC BY-NC, is still in use, but they used to also have the Sampling+ license as an option until that was retired by Creative Commons. Some old sounds are still under that license, though.)
