I am part way through building yet another arduino based synthesizer.
Not as fancy a box as others yet, but I am quite happy with the software. I am designing it as a self-contained toddler proof synth, for my daughter to play with, hence the lack of moving parts / switches outside the box, speaker inside the box etc.
Roughly 32khz output sample rate.
8 voice polyphonic oscillators (each with independent pitch and volume), based on 256 sample 8 bit wavetables.
Square, sine, sawtooth, triangle waves
One resonant low pass filter (a dirty hack of a filter, based on the filter from the meeblip open source synth).
8 Capacitive touch sensing input 'keys' so you can play chords easily.
All the processor intensive bits written in assembly and very heavily optimised in order for it to work without maxing out the arduino.
A not quite up to date, but making a nice noise video. It now sounds a fair bit better thanks to 'Grumpymike' from here, who fixed my amplifier problems.
More information and code here:http://www.cs.nott.ac.uk/~jqm/?p=605