Hi mac music,
I have been looking for a simple program for years which will recognize an audio signal from a bass etc... and send an appropriate midi signal to an outboard synth or the internal sequencer.
I know there have been for years midi interfaces for guitar and some complex environments like max msp, but all these are so expensive and far more than I need. Searches always throw up tons of cheap or shareware PC programs but none for OS X or Unix. Someone must have something somewhere.

this is a shareware vst plug, but its OS9 only:
http://www.hitsquad.com/smm/programs/PI2M/

thats the cheapest i know of. melodyne will also do what you need, on OSX too, but its not cheap. Keep checking every monday on OSXaudio.com (i think thats when they update) cos an app like this is bound to turn up eventually.

contacted melodyne alas they say it doesn't do what i want, only works with recorded files, not live input. still amazes me that with all this software no one makes the equivalent to midi controller for a guitar, technology that has been in hardware for ten years or more. i will keep looking, i guess.

Try Kantos

Kantos doesn't do pitch to MIDI; rather, it does aggressive manipulation of sound to give synthlike sounds from vocals or other rich audio input...

Have looked into vocals to midi, on dedicted hardware, PC and Mac, but never found a useable solution for realtime use.

Best solution I've tried is the old Roland VP70; best apparent solution is a device called MidiVox, based on reviews. Google will find it... On VP70, problem is response time and tracking accuracy.

Given the low price of PC's, and the amount of processing required for real time pitch tracking, most interesting option is one of the PC based programs, but I've never had much luck with them, even tho several claim to work well for this app.

Graphtech make piezo saddles which convert what your playing into midi, you can then connect your guitar to a Roland V system (or other similar midi brain) and then the midi out of that into your computer.

I'm considering doing something like this with an Ibanez RG421

For what it's worth, I bought a Roland GM-70 guitar-midi converter with a GK-1 pickup, and it works marvelously. It was $100, and I've seen similar prices on eBay. That may be the best option for a guitarist on a budget. I understand tracking is pretty comparable to new units, and it definitely would work well for most basslines, IMO.