Windows/Mac OS X: BlackBerry Desktop Software effortlessly syncs your BlackBerry with your Mac or PC, including music, contacts, and appointments. The Windows version isn't new, but the app just hit OS X today.
I've got a Mac and a PC but no BlackBerry, so I wasn't able to test it with any gear, but the gadget-obsessives over at Gizmodo have BlackBerrys coming out their ears, and they like the new BlackBerry Desktop Manager for Mac even better than the Windows version.
They both do basic syncing for the information you'd expect. Apart from syncing, you can back up everything on your phone to restore it later in case anything goes wrong. The Mac version can sync music to your device using iTunes playlists, while the Windows version syncs music via some Roxio software (which is one of Gizmodo's big problems with the Windows version).
Still, whether you're on Windows or OS X, if you've got a BlackBerry you'll probably want to grab this app.