There is a new release of ScummVM out. The adventure game engine that brought classic Lucasarts titles to Linux and other modern operating systems, now supports Sierra and other AGI based games through the latest version 0.X.0 (or 0.10.0 in standard numerical format). This brings a plethora of freeware titles, that previously could only be run on Windows or through WINE, to Linux.
There are a lot of AGI fan games out there. A lot. Of course not all are high quality but there are some gems out there. Since the games themselves don't tend to be open source I'm not going to delve too much into it so if you know any cool AGI games please comment and share them.
I was under the false impression that AGD's King's Quest remakes were AGI games, when they are actually AGS games. So you won't be playing those natively on a non-Windows system any time soon. :-(
Too many AG* acronyms... anyway, briefly back to the AGI stuff and here's an article giving an overview of AGI code. It looks pretty obvious so any budding adventure game makers - and there are few around, this is a dying genre :-( - might want to look there.
Somebody challenged themselves to come up with an original game mechanic/concept per day for 300 days. Some of the ideas are dubious at best but still a good source of inspiration.
Finally, Julius in the FG forums posted a few more sound resource links, so there's no excuse for creating quiet Free games any more since there's so many options for you to give your game some ear candy.