CyanogenMod (CM) has long been the modding community's custom ROM of choice, and for good reason: it takes the goodness that is basic Android (AOSP) and adds a hearty dose of all-around improvement. The list of devices supported by CM is quite a long and impressive one (truth be told, they do a better job of supporting devices than the manufacturers do), and last night, cutting-edge nightly CM9 releases dropped for the Sony Xperia Arc (Anzu) and Xperia Neo (Hallon).
It could oftentimes be unstable and not properly tested, lacking any changelogs, but eventually evolving into alphas, betas, release candidates, and finally stable releases.
No word yet on just how usable the builds are, but this is good news for Arc and Neo owners, as it means the devices are actively being supported at this point. Plus, who can complain about AOSPy Ice Cream Sandwich goodness?