UOX3 is written in C/C++, it uses the Spidermonkey Javascripting-engine as an event-driven scripting language.
It is named Ultima Offline eXperiment 3 because it was the first publicly available UO emulator, and at the time of release it didn't have multiplayer support, but was only meant to allow you to run explore the UO world locally on your own computer. UOX2 came and went, before UOX3 arrived- which is the version the current version has evolved from.
As for features... it works with the Iris client

At least, did the last time I checked - which was a while ago, granted. Features? Hm. UOX3 is open source (most are, these days), compiles (and runs) on Linux as far as I know, has a scripting engine which is pretty decent in capability and yet doesn't use a language which alienates anyone who can't code. *shrug*