Today I ran accross this free program called MusicPublisher that lets you share a folder or an iPod to iTunes via DAAP (the iTunes sharing protocol). This works perfectly for playing files from my many music folders not stored on my iBook's hard drive, or for files I want to listen to once and then delete -- all from within iTunes. Everything is instantaneous, and memory and CPU usage are both very low.
[robg adds: I haven't tested this program, but I have been asked about such solutions before. My prior answer has been to either add, listen, delete in iTunes (a pain), or listen in the Finder (no playlists, etc...). This seems like a much better solution to the problem.]

