I've been playing over the weekend with Rax 2, a MIDI host program for Apple audio instruments and effects. I first bought Rax when it was put out by Granted Software, who have made a number of great, inexpensive programs (including ReVision and Still Life). Anyway, Rax has been taken on board by Plasq who have significantly upgraded it.
What does Rax do? Well, connect your MIDI keyboard to your computer (I use a Tascam US-428 at home or a Roland UM-1 if away). Fire up Rax and you're presented with a multi-channel mixer. You can load in any Audio Unit instrument and play it using your MIDI keyboard. You can assign different instruments to different channels and adjust their levels. (I haven't gotten quite as far as trying multiple istruments at once yet, but that's the whole idea. There are plenty of AU instruments out there.
Then you can also add in Audio Unit effects, and again there are a stack of those available from Apple and others.
Rax includes the option of recording your songs for later playback live. This is through a new Onstage panel which I'm still figuring out. You can have multiple racks of sound libraries with multiple instruments and multiple songs.You can drop in an existing MIDI file and play along. All sounds great if I can figure it out.
Another new feature is a number of Quartz visualisations which (presumably) can be ported to a data projector (I don't have one handy at the moment to test) and have elements triggered by the MIDI keyboard. So you get a live performance visualisation as well. Some of the visualisers are excellent and some are OK or a bit weird. Plus you can have a go at making your own or editing other people's!
Apparently you need to use the Quartz Composer tool which is part of the OS X Developer tools. And there are apprently plenty of other people's compositions. eg. here.
The plasq website has a set of free screensavers for Mac based on these visualisers. They're great.
Overall it seems brilliant. I bought the earlier Rax for US$20 and now its US$50, but still great value. I'll be happily playing some more tonight.