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Precisionsound - "High quality sound samples for music production and sound effects for the multimedia/movie industry" - material in various formats, mostly for purchase.
ThaLOOPS - professional Hip Hop audio samples and loops for music production. Download Hip Hop Samples, Hip Hop Loops, Free Drum loops, acid loops. Professional and unique sound libraries for Hip-hop, R&B and Pop music production. Variety of downloadable cutting edge sounds. FREE content available for tryout!
PeaceLoveProductions, PLP provides a destination to download royalty free music loops, beats, and samples for use in any music software
ModernBeats offers acid loops and drum samples for urban music production, plus beat contests, music software, and music contracts for modern musicians
Loopology content for Adobe Audition, "thousands of 32-bit, uncompressed, royalty-free audio loop files in over 20 different musical styles that range from rockabilly to cocktail jazz and classical to funk." NOTE: You do not need Adobe Audition to use these loops.
MPCsounds offers music samples& loops for musicians that use the Akai MPC drum machines for composing music.
Flash Kit SoundFX, "Flash Kit - an online resource for Macromedia Flash development. Join in our message board discussions, read the latest Flash news and tutorials, and download our free FLAs, SWFs, and sound loops."
Equinox Sounds, developers of loop/phrase libraries in ACID/WAV and REX2 formats. Royalty free loops for electronic music producers.