Well, Max/MSP, which I've discussed extensively in
another thread.
On the music/audio front, I've also been getting a lot of use out of SoundHack recently - a great free tool for combining sounds to create beautiful drones and atmospheres.
But retro-gaming, now that's another matter. I use - and though it shames me to admit it, love:
MacMAME - for playing vintage video game ROMs on the Mac
ZXSP - crazy Spectrum "simulator", which actually requires the user to load games from virtual tapes!
SMS Plus - Sega Master System emulator