There is a plethora of software options available for the PC Engines ALIX board. A sufficiently long list of both free and not-so-free operating systems is maintained on the PC Engines website here.
However there are other versions of software not on the list that are available to you if you don’t mind a bit of tweaking. For example Lyle Scott recently pointed me to his blog, on digitalfoo.net. One of the interesting tweaks he discusses at length is porting NanoBSD to the ALIX platform.
NanoBSD is a subset of scripts in the FreeBSD source tree that enable you to run FreeBSD on an embedded device…such as the ALIX board. I know Lyle isn’t the first person to apply FreeBSD to the ALIX board but he has done a great job of laying out how it is done. Check out his post: Creating a NanoBSD Access Point (AP) and Router here.