what good music composition web apps are out there?
SwellJoe: Nowhere.  Latency is just too high with Flash or JavaScript or Java applets, even the best online apps are atrocious.  I always tinker with them when I see a link to them, but they always disappoint.  Most iPhone and Android music apps are also pretty much useless, unfortunately...I love composing in odd places.There are many good free (and low cost) installable apps, though.On Linux, there's LMMS, Ardour, Audacity, which all do different things...I'm guessing LMMS is the one that fits your description best.On Windows, there's FL Studio, and dozens of others.On Mac OS X, I dunno the low cost options, but I'm sure they exist, since Mac is a very heavily used music platform.My toolchain contains the following software:  REAPER
  Renoise (on both Linux and Windows)
  LSDJ (on Gameboy)
  MSSIAH (on C64 with a SID2SID)

I will probably add FL Studio to the bunch eventually, but I mostly do live music and chiptunes, so it's not something I really need.  But it's cheap enough to buy just to tinker with.