Mamesick does a good job listing features for MAMEUIFX. It's basically MAMEUI with a lot of nice features and some patches. It uses the same ROM set as MAME.
MAMEPlus is also a nice build. It add some MESS console drivers and also International support. There are a lot of other features, but it's hard to find an entire list. There are a few extra ROMS added mostly from MESS. Also take a look at MAMEPlusGUI.
MAME Plus Plus is an "illegal" build, as it volatiles the MAME license. Not sure of the details built I think it has network gaming support and has Zinc drivers.
Also take a look at MAMEUIXT. EmuLAB has a newer build. It's based on MAMEUIPlus and/or MAMEUIFX. Adds a lot of artwork support.