Threads like these don't get made often, but when there's something that comes around and strikes me as "wow, this is kinda awesome", I post!
FFXI Calculator is an application (separate from the game, no hooks involved) that can be used in a variety of ways to calculate different things. Not going to sit here and try to go through every feature, so just download it for yourself and see what's in there. On top of being pretty clutch in the calculation department, it's a good source of information for things like Dynamis, Limbus, and Assaults. However, one of the things that this application does that I found to be of best use is the ability to regedit FFXI without having to actually go into the registry and do said tweaks. There's a clear-as-day GUI that lets you edit it however you want, thus letting even the stupidest people make their game look grand. People are always asking how to do it, and often get lost/confused/screw something up when they're playing around in the registry, so use this for that if for anything at all.
"I only used Windower!"