Greg Squire
Well the big three engines have been mentioned

Adventure Game Studio (AGS)

All of those are great engines. Visionaire is probably the best to use for a commercial game as it is now cross platform, well supported and is used by Daedalic Entertainment (a big adveture game studio in Germany). AGS is a good second, the code base is open source and community supported. Also lots of commercial indie games made with it. Wintermute is also good but still Windows only.

The plug-in for Unity called Aventure Creator (mentioned previously - also looks promising. Unity3D is used by tons of developers, is highly supported and cross platform.