Partners In Rhyme has a good set of Free Sound Effects and links to many more. Samples can be copyright-free, with limitations, or royalty free so check the facts. See also www.ljudo.com and SampleNet. If you're in a bind use FindSounds to search the Web for lo-fi sounds. There's a ton more of free sounds available on the net, always needed by Flash artists, so Google away.
Update: DV Guru noted the freesound project which has teamed up with ccMixter to "share information about freesound's 20,000 samples and track remixes across the two sites. It's all part of the Creative Commons Sample Pool and therefore available to all remixers, safe, legal and sane."