Well, somebody just released an OSX rewrite of SoundApp, which would have been on my list (although frankly with iTunes as good as it is now the only reason I was interested in it was its ability to play weird-ass sound formats).
Other than that... Super Tetris and Loom. X-Wing would be awesome too, but only if it was a full-on re-release with slightly beefed up graphics.
Ooh, and Sam and Max! Man, LucasArts has SO lost its way...
Those, and QT plugins for Indeo 3.2, 4, and 5 video--VLC seems to be edging toward support, but nothing functional yet. That's really the main reason I keep Classic around these days, actually.