If you think the Mac world doesn't have room for another DAW, Harrison Consoles (harrisonconsoles.com) is out to change your mind with Mixbus ($79.99). Its low price belies its full feature set, which combines the open-source Ardour DAW (ardour.org) with a mixing console created by the designers of Harrison's high-end digital consoles for the film, recording and broadcast industries. Each channel strip — input, track playback, bus and master — sports a 3-band EQ, a highpass filter and a compressor with Level, Compress and Limit modes. The compressors on the four bus-channels also offer sidechain inputs. As Harrison's Ben Loftis puts it, this is not an emulation of Harrison's DSP; this is Harrison's DSP.
In addition to such handy features as time stretching, pitch shifting and varispeed playback; the Rhythm Ferret for transient slicing; real-time transition between forward and reverse playback; and the usual DSP operations, you'll also find some of the quirks that often haunt open-source software. Audio routing requires the free and included Jack Audio Connection Kit (Jack OS X), and that entails a bit of setup. AU plug-ins are supported, but their graphics are sometimes iffy. The documentation is sketchy, but combined with a little DAW experience, there's enough to figure out most features.
Although Mixbus is not likely to become your only DAW (for one thing, it doesn't host virtual instruments or offer MIDI sequencing), it is a worthy addition to your kit. Its low-CPU footprint makes it ideal for live recording on a laptop. You can import stems of MIDI and audio tracks from your main DAW into Mixbus, and then use its straightforward yet powerful mixer for editing and post-production in the studio or on the road. And Mixbus is a great tool for sketching and remixing with files in your audio library (see Web Clips 1 and 2).