Never knew a sequel was due out! I thought that it was always going to be left on a cliff-hanger. Although I think I remember reading on a forum it could not of been any of the main characters for various reasons.
Broken sword series 1 + 2 (even more so number 1), are very good. Such a shame they went the '3d' route after 2. The game lost so much of it's soul.
And of course, no adventure list is complete without the obvious Lucas Arts classics. Especially Monkey Island 2 + Indiana Jones Fate Of Atlantis.
If you're a newcomer to adventure games then your in for a treat really. There are not that many games of of the genre being released now really and even less-so of any quality. But, there was a time when they were so you have a lot of catching up to do.

Also, you should give 'Planescape: Torment' a try. It is an RPG but im sure you'd like it as the story and exploration is fantastic.