Existing users of OS X will be able to upgrade to 10.1 for free via an “instant update” kit available in participating retail stores this Saturday (both Apple and participating resellers). Offer good through October or while supplies last.

What made the cut for 10.1? DVD playback, CD and DVD-R burning, better AppleScript, easier printing and printer support, better digital camera support than any other OS anywhere, better 3D / OpenGL support, ColorSync, and the long-awaited multi-channel, high quality (32-bit/96-kHz), low-latency audio/MIDI support, all built in. Plus much-publicized speed improvements and interface upgrades. Apple’s press release has more.