As you may know, before Avernum, Geneforge and Avadon, Spiderweb Software's Jeff Vogel made the Exile series, and some continue to regard it as better and more complex mechanics-wise than his later efforts. Thankfully, all three games in the series have recently been made freeware and can be downloaded from the Spiderweb website, so now you can check them out for yourself:

Exile: Escape from the Pit
Exile II: Crystal Souls
Exile III: Ruined World

(I'm linking the Windows versions, but you can also find the links to Mac versions on this page.)

The games should run on any pre-Vista Windows system, but you may need to use Virtual PC to make them work on Vista or 7.

Thanks Awor Szurkrarz!