Numerology 2.0 ($119) from software developer Five12 is a modular, multitrack, step-sequencing workstation. It is a standalone program that boasts its own sample looper and sampled drum machine, and it hosts AU instrument and effects plug-ins. You can also link and sync it to your DAW to use just its step-sequencing capabilities, which are prodigious. It will run as a ReWire slave, but I find it more convenient to sync Numerology using MIDI Clock and then use its MIDI ports to route data back and forth. If you use Numerology's audio features, you can pipe its audio output to your DAW with third-party audio-routing software like Cycling '74 Soundflower (soundflower.com; free).
Numerology has many tricks up its sleeve for generating note and MIDI continuous controller sequences. Rather than applying one basic design to different sequencing objectives, it has separate note-sequencing modules designed for monophonic, polyphonic, drum, and arpeggiation-style sequencing. Beyond that, you can get under the hood and build your own sequencer setups. That method, called discrete sequencing, is one of its best options — it lets you manipulate the Velocity, trigger, and pitch sequences separately. You can, for example, change their lengths and playback directions independently and even automate those changes to create long-running, but not completely random, patterns (see Web Clip 1).
Besides note sequencing, you get a variety of CV (control voltage) generating and processing modules such as LFOs, envelope generators, virtual manual controls (fader, joystick), parameter modifiers, and signal analyzers. Internally these operate at a higher resolution than MIDI and are then converted to standard MIDI continuous controller messages at the output. You also get sophisticated clocking options for simultaneously altering the groove of all your step-sequencer modules. You can download the full version of Numerology from the Five12 Web site (five12.com), try it out in save-limited demo mode, then buy it online if you like the results.