Suppose you have a nice Ant script for deploying your software. As a developer that's great, but your poor user might not have Ant installed on their machine, and even if they do, a text-based installation script isn't exactly friendly. Antigen will allow you to reuse your script, with all of Ant's power, and wrap it all up in a nice friendly GUI. The user doesn't need to install Ant, they just double-click on the executable jar file that Antigen creates.
Ant Installer GENerator. I wanted InstAnt, but someone had already bagged that. Plus, it has more scope for sad puns.
A pollen grain. More of an allergen, but I guess it must be covered in antigens since the antihistamine industry is so profitable.