I was looking for some free singing speech-synthesis software the other day, reading this post from EM411. There doesn't seem to be much for PC, but Vocal Writer does the job very nicely for free ($39 shareware), and it's just been updated to OSX. Listening to this [mp3] sample of the thing singing 'Penny Lane' made my day, and I spent some time fiddling with it at work. It sounds almost exactly like a very badly edited spoken-word recording fed into a vocoder.