Free Music Software

U-No-62 is free virtual Juno synth, available in AU and VST formats.
Features:

More accurate filter sound.
More overtones in the oscillators.
Faster LFO rate (0.1-30HZ).
A lot faster envelopes.
Audio Unit available

Beatesthesia is a highly customizable WYSIWYG music visualizer, inspired in motive and design by synesthesia.
It’s a free download, but is currently in beta form, so there’s a bit of a learning curve and you may encounter some weirdness.
Description:
Synesthesia has inspired many works of art. Art that is meant to evoke synesthetic associations in a non-synesthetic [...]

IK Multimedia has announced that AmpliTube X-GEAR has been made available as a free download for AmpliTube 2 Live users and StealthPlug users.
X-GEAR is an expandable guitar amp and effects modeling software system, available as a plug-in and standalone version for Mac/PC, that allows mixing and matching between all the gear models in a single [...]

Cynthia is a free Windows VSTi/VST inspired by the classic EMS Synthi A.
Specifications (like Synthi A):

3 oscillators with controls for shape and level of the waveforms

OSC1: Range from 0.6 Hz to 16 kHz, sine and triangle
OSC2: Range from 0.6 Hz to 16 kHz, square and triangle
OSC3: Range from 0.015 Hz to 500 Hz, square and [...]

Wusik has released its Wusik VM VST Manager/Organizer as a free and open source music project.
I think this may be the reason why they are having to turn this thing loose:
The idea behind WVM is very simple. You load WVM and inside it, you load your VSTs. Once you extract the presets to FXP files, [...]

Got a fever for more cowbell?
Dynamite cowbell, a free Windows VST, brings you “more cowbell than even Gene Frenkle can deliver”.
Features :

6 cowbells mapped to midi keys
5 velocity layers
close and room mic levels
damping control
5 bonus digibells
1 cow

Gnaural is a multi-platform programmable binaural-beat generator, implementing the principle of binaural beats as described in the October 1973 Scientific American article Auditory Beats in the Brain (Gerald Oster).
It can be used to synthesize sounds that affect the brain. Here’s a demo. It should be listened to using headphones:

 

 Gnaural [73:30m]: Play Now | Play in Popup | Download

pfilter is a free Windows VST that creates a steppy, gated effect, but using filter cut-off instead of volume to give a wider range of possibilities. It uses a variable state filter, triggered between 2 adjustable cut-off values by a tempo-sync’d probability-based step sequencer. An LFO can modulate the filter when triggered and trigger pulse [...]

KarmaFX has released Version 2.0 of the WIndows VST KarmaFX Plugin Pack, which features reverb, delay, EQ and Filter plug-ins.
KarmaFX Plugin Pack 2.0 changes:

Added performance improvements.
Added SSE3 code path and memory optimizations to Reverb plugin.
Plugin dll’s are now compressed for faster load times.
Individual version numbers for the plugins have been removed.
The plugin pack now [...]

Moo Cow Music’s Band lets you record tunes on your iPhone or iPod Touch.
Do you think these iPhone apps are anything more than music toys?

terminatorX is a realtime audio synthesizer that allows you to “scratch” on digitally sampled audio data (*.wav, *.au, *.ogg, *.mp3, etc.) the way hiphop-DJs scratch on vinyl records.
It features multiple turntables, realtime effects (buit-in as well as LADSPA plugin effects), a sequencer and MIDI interface - all accessible through a gtk+ GUI. This software is [...]

Glitch is a genre of electronic music derived from a fascination with the sound of technology failing in interesting ways. While the music originated with musicians exploring things like CDs skipping, the sound of glitch has been absorbed into all types of electronic music, with musicians using glitch noises and effects within a rhythmic framework.
Here [...]

This iPhone synthesizer video is making me lookforward to iPhone apps in iTunes.
FYI - this is just the latest of many iPhone music software stories.

NUSofting has released Groove Analogizer, an audio controlled drum synthesizer, as freeware.
Similar to a vintage electronic drum tone generator (e.g. Simmons) the percussion sound is triggered by an audio input, not by a MIDI message.
Groove Analogizer features:

3 oscillators synth (loads as effect plug-in), triggered and shaped by the audio input in many ways.
Selection of left, [...]

Mario Paint Composer is a remake of the music system from the original Mario Paint for the Super NES. It is a free music application made by unFun Games. It’s available for both Windows and Mac.
It’s a pretty limited app - but that’s part of the [...]