In constrast with Pure Data, Tcl can do things with lists quite simple.
This is more a demonstration of what can be done with lists in Tcl than a real arpeggiator. It accepts midi and float lists with key and velocity. It plays the notes in the order of arrival, removing (velocity = 0) and appending (velocity > 0) on the fly.
Fun to make and a nice demonstration of various tclpd features. More info on tclpd at http://fjkraan.home.xs4all.nl/digaud/puredata/tclPd/