former PLT scheme = Racket...

it's a tight little platform! batteries included... check out DrRacket, and it's dependency graphs that appear upon mousing over defined words...it will even show graphics and mathematical symbols in the REPL if that happens to the be the domain of the functionality you are testing... loads of libraries available... great package system. there's a pretty basic thread-per-connection web server (extension of a generic server i forgot the name of) included and you can get up and running building something very very quickly...

OS X ships with a couple hundred files worth of Scheme related to sandboxing:

    mdfind -name .sb

lilypond is a big one. Better music notation quality than finale and sibelius.