Humans, Slithzerikai, and Nephilim for Avernum 2-6 (Blades of Avernum too). The only games that dont allow you to use the races is Avernum and Avernum's remake of it Avernum Escape from the Pit. That game isn't included with the bundle.

The races are as follows-Humans get no experience penalty and are usually better off wielding swords and or being a mage or priest.

Then comes Slith. They are deadly with spears and hardy against fire. They also have a health increase in Avernum 5 and 6.

Then finally my favorite race, Nephilim. These guys are like the rogues of the games. They are hard to hit and good with a bow. Because of that they generally can be either a archer, which is a good deal. Or they can be a finesse melee warrior who wields swords. I usually give them a sword and pump dexterity and other skills.

The cool thing about the races is that you can make the characters any way you want regardless of what race they are.