Virsyn Software synthetiser announces the release of Virsyn Tera for Mac.
This modular software synthesizer (based on five different synthesis technologies including the "Spectrum synthesis") allows the creation and manipulation of resonant bodies with up to 128 formant frequencies. The user interface of VirSyn TERA is said by its developpers to set a new standard in design and functionality. Thus simplifying the access to the huge sound space of its predecessor(Virzyn 1.1).
Shipment date for the Mac version is August, 26th.

BitHeadz, Inc., the music software manufacturer has released a new update for the Unity product line.
The 3.0.4 update will provide a number of improvements and features to the current owners of Unity Session, Unity DS-1 and Unity AS-1 version 3.0.0 and up. It will be available as a free download for all registered owners of any of these programs.

Unity 3.0.4 will introduce support for the MP3 file format. Users will be able to import them as samples just as they currently can with AIFF, .wav and Sound Designer II files. The imported MP3's will then be fully editable with all of the features provided in the Unity Editor.

Another addition to Unity version 3.0.4 is support for VST clock, which allows users to sync the tempo of any loops loaded into Unity to their VST compatible sequencers.
Some other extra features are improved OSX menu support, better bank loading, and restored support for Mac OS 8.6 and 9.0.

You probably knew steinberg's virtual guitarist; here's Slayer, the virtual guitar.No samples but a true VSt instrument based on physically modelised synthesis.
Slayer emulates the guitar, the amp, the cabinet and the FX rack.
Note that the bass isn't forgotten, as well as some ethnic instruments.
Demo and info on ReFX server.

Richard Bannister release a shareware, Cacophony which is able to edit multichannel audio under X

If you need a metronome under MacOS X you can use MetroGnome a little freeware rather practical.