I have some hope for the adventure genre yet, even after being seriously let down by Monkey Island 4 (make sure you've played Monkey Island 3 and Grim Fandango though; they're both excellent!)

That said, I've heard Psychonauts is good, though I haven't played it. My understanding is it's free on Gametap.

One that I missed when I was growing up was Touche: The Adventures of the 5th Musketeer - it's just about as good as the Lucasarts games - Note: there is a version with full voice acting, and this is the only place I know where you can actually get it: http://thepiratebay.org/torrent/3557929/Touche_-_The_adventu...

I haven't played this, but it's supposed to be very good as well (made from ex Lucasarts members): http://www.vampyrestory-game.com/

I found the Sam and Max series from Telltale very underwhelming and disapointing, so I never played past episode 4 in the first season.

On a somewhat related genre note, I've also found the game Beyond Good and Evil to be as immersive and entertaining as the MI series.