Which one?
There were several OSs.
MER, an old Android port, the one it shipped with(dont recall), and two or three others at a minimum.
FWIW it only does 2g and even that only actually received calls say 70% of the time.
I can look for the battery for mine, it should be in a desk drawer with the Neo Freerunner.
Edit:
http://wiki.openmoko.org/wiki/Download
Looks like nothing past 2009...
I really gave the Freerunner a good try before I bought a N900, it was never really a reliable daily use phone though. There was never a large developed app ecosystem available for the 1973 and freerunner,(phone, sms, gps-OSM, maybe email, terminal, old IMs, a clock, mp3, wifi, bluetooth but that all required terminal for pairing and settign up conf files and scripts) the Maemo repos were overflowing compared to the few dozen apps for most of the various openmoko OSs. I hope the Pinephone works out better for a daily user!
Edit 2:
I see our Joerg was the last guy to edit the moko front page in 2013!