Cockos Reaper might not have the greatest name ever, but it’s an impressive audio/MIDI Digital Audio Workstation.
It’s relatively inexpensive, $225, or $50 for non-commercial use. It runs on Windows 98/ME/NT/2k/XP/Vista/WINE, and a OS X version is on the way.
A new version, Reaper v2.45, is available.
Here’s what’s changed:

Tracks can be set to automatically record-arm when [...]

Chris Randall continues with his video overviews of Audio Damage sofware with this introduction to Dr. Damage.
Dr. Damage Features:

Multi-mode filter (4PLP, 3PLP, 2PLP, 4PHP, 2PHP, and 4PBP modes). The 4PLP self-oscillates at high resonance without audio input, allowing Dr. Device to be used as a theremin-type instrument.
“Tube” style soft saturation and bit reduction.
Stereo bucket-brigade-style delay, [...]

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 [...]

Schaack Audio Technologies has announced the release of version 2.0 of Quad for Windows (XP, Vista) and Mac OS X.
Quad is an open-architecture DVS (Digital Vinyl System) that can be used to play & scratch up to 4 MP3s simultaneously, controlled by 1 to 4 turntables. The turntables can be connected to any soundcard (ASIO [...]

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 [...]

SimpleSynth is exactly what you’d think it would be, a no-frills Mac software synth designed for when you just want to play without the hassle of firing up and configuring your favorite sequencing software.
It’s got a complete general MIDI instrument set built in, and it can load Soundfonts and DLS files.
Don’t plan on doing any [...]

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 [...]

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 [...]

Techspansion has introduced AudialHub — an inexpensive Mac app designed to convert dozens of different audio formats (and even the audio tracks from video files) to AAC, MP3, WMA, WAV and Apple Lossless formats, with the standard metadata tags in most formats converted and passed on to the finished file.
Nearly every aspect of the decoding [...]