I second Fabio's choices (though I've never played Syberia... perhaps I should) and would certainly recommend
Monkey Island 1 & 2 if you enjoyed Sam & Max. The new "Tales" aren't bad either, but not quite on the level of the originals.
Curse of Monkey Island (the third game in the series) is excellent, if you can actually track it down. Same goes for
Grim Fandango if you can track it down
and make it work.
Oh, and make sure you download
Scumm VM. It enables you to play loads of old freeware and abandonware games, not least the excellent
Beneath a Steel Sky and very enjoyable
Flight of the Amazon Queen which are available for download on the same page as the installer.