Never used FreeNAS (or had a system that would utilize it) but ArchLinuxARM is nearly limitless with what you can do with it. The devs here made it simple to set up, even host guides on settings up tools like Samba and Transmission. Arch is relatively easy to pick up even if you've never been one to work with Unix.
Depending you which plug you have (look at the bottom, find model saying either E02 (ARMv5) or P01/P21/V3/B04/B05 (ARMv6)), check the front page under Platforms to start installing. Then check Support->Guides->Applications for setting up Samba (if you want to network Windows through file browsing through the NAS) and Transmission (REAL easy to use, or if you fancy other torrents, the repository has rtorrent/libtorrent).
You can obviously do more with your plug after this point. Check through the forums to see what other people are doing. Popular stuff include using MiniDLNA/Subsonic/Twonky/MediaTomb to stream music and movies to devices, network a non-networking printer through CUPS, host a website with the usual combo of Apache, MySQL, and PHP, and bunch of other things. If you are familiar with Arch or Unix in general visit
The ARM Repo for all the available packages ready to go.
Like I said, I never used FreeNAS, but from what I've seen it is very limited. To keep your pogoplug open to different things give ALARM a shot. Best part, if you don't like it, devs made a hand revert script, too (doubt you'll need it though

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Cheers, and good luck!
-Sly
Post script: I am not a paid sponsor, just want to help you with that final push to try it out
