This Replicant screencast by Chris Randall demos the latest sound mangling software from Audio Damage.
Here’s the Replicant description:
Replicant is a delay-based effect in the “beat slicer/looper” vein, with some tricks of its own that separate it from the herd. From a simple delay or panning effect, on up to massive random beat mangling, Replicant is [...]

Audiofile Engineering has has announced Rax 3 - the latest version of its virtual rack software for OS X, and has published a video to demo the updated software.
New features include new input sources, a completely redesigned user interface, radical routing capabilities, and liberating MIDI mapping opportunities.

DJ-1800, a mac DJ application, has added new features with its v3.2 update, including the ability to stream DJ mixes to the Internet!
New features:

A new full-screen mode
Full integration with Nicecast from Rogue Amoeba, to turn DJ-1800 into an internet radio station
Support for the Hercules DJ Console Rmx
Many other improvements and fixes

Nicecast and Hercules DJ Controller [...]

DJ-1800, the pro DJ software for Mac, has been updated to let you use music stored on the iPod touch and iPhone.
Features:

Support for music stored on an iPhone or iPod Touch
Support for new DJ controllers, including the Denon DN-HC4500, M-Audio X-Session Pro, Numark DMC2, Numark iCDX and Numark Total Control
A full plug-in API, enabling you [...]

InstantSampler is a freeware simple sampler for Mac and Windows that records audio from the audio input when a MIDI note is played for the first time.
Any consecutive time you play the same note, it plays back the recorded audio with the possibility to vary or trigger:

Velocity.
Attack and release.
The level of the input [...]

Bitter Sweet II is a freeware dynamic processor for managing audio transients, available in RTAS VST & AU formats for Windows & Mac.
Description:
Bitter Sweet II is especially active on loud percussive tracks.
Turning the central button on the sweet side, decreases the transients amplitude. On the bitter side, the transients amplitude is magnified.
Bitter-Sweet also features a [...]

Sonic Visualizer, a free application designed to make exploring audio data as fun as possible, has been updated.
Sonic Visualiser will be of particular interest to musicologists, archivists, signal-processing researchers and anyone else looking for a friendly way to take a look at what lies inside the audio file.
Sonic Visualiser has powerful annotation capabilities to help [...]

MixMeister has announced MixMeister Fusion and MixMeister Studio 7.2. This release marks the first Mac versions of MixMeister.
Version 7.2 of both packages includes the following:

Native Mac OSX version is universal binary for PowerPC or Intel processors
Improved time stretching accurately matches beats with fast or slow tempos
Enhanced timeline display provides clear division of measures
Effect automation via [...]

M-Audio has released a new FireWire driver for Mac OS X Leopard 10.5.2.
This driver update is available immediately for the following devices :

FireWire 1814
FireWire 410
FireWire Audiophile
FireWire Solo
NRV10
Ozonic

This driver does not support ProjectMix I/O or ProFire Lightbridge.

OT: This music video, for the song Again and Again by Bird & the Bee, makes really clever use of various features of Mac OS X, creating a constantly changing visual interpretation of the music, despite limited resources. Very cool!