For editing UI XML specifically, I use Visual Studio. If you point it to the UI.xsd file in FrameXML, then it knows how to autocomplete WoW UI XML. It knows valid values for anchors, which tags fit where, and stuff like that. All that should work in any of the free versions of Visual Studio like the Express editions (
http://msdn.microsoft.com/express ), or the Add-on Studio for World of Warcraft version that's specifically made for WoW mods (
http://codeplex.com/WarcraftAddOnStudio ) Plus, you'll get a listing of all of the errors in your XML, and they'll be highlighted with nice little red squiggles like spelling errors in Word.