I've always been a big fan of [Propellerhead->http://www.propellerheads.se]'s Rebirth RB-338 software. I remember how amazing it felt back in '96-'97 cranking out legit-sounding beats and blips on my Powerbook 5300 in the van while on tour with Polara. Rebirth was certainly the first useful virtual instrument I ever ran on a portable computer; Rebirth plus the late-lamented Studio Vision Pro were the very height of "in the box" tech in those days. Propellerheads have now discontinued development of Rebirth, but in consideration of the historical importance of the program they've launched a [Rebirth Museum website->http://www.rebirthmuseum.com]. It seems a bit absurd to have a "museum" dedicated to a piece of software, but the site does a great job of explaining the roots of the instrument (it was modeled on Roland's TR808 and TR303 drum machine and bass synth) and putting some perspective on just how much a breakthrough it was. There really had never been such an efficiently-performing software instrument that sounded like the real thing yet was affordable to normal people. And Rebirth also looked very cool, adopting the look and UI functionality of those familiar Roland devices. These days every [Apple->http://www.apple.com] computer ships with Garageband, an incredibly flexible sound-generating application. It's humbling to think where computer music was at a scant ten years ago.