sippan: try Dimension3 (dim3)
http://www.klinksoftware.com
http://www.idevgames.com/dim3

its FREE and in my opinion, the easist and best 3d game engine.

I was actually thinking about trying to make an adventure game myself.

Adventure Game Studio is a good bet.

Downside: It is "Windows only." That said, the editor runs in Parallels and Darwine just fine.

The runtime engine is another thing. There is a beta version of the Mac runtime that will run some games (like the Tierra games) just fine. But it won't run games made with the latest editor... yet. From what I've read, porting isn't much of a problem since it uses a cross platform library. But its not open source, so they're reliant on a single porter.

If you want to write SCUMM style games, I strongly suggest you look into it.

I would suggest checking Torque Game Builder out. Feel free to download the trial and give it a shot. If it feels like something that would work well we even offer an Adventure Kit

TADS looks cool - maybe give it a try? It even does multimedia.

http://www.tads.org/