As far as "in game GPS" goes, your map already does that. If you type <pos> the game will display your map coordinates.

However, as far as mods go, there aren't really tons. There's model viewer, but that doesn't really affect the game at all. Then of course there's the windower. The windower is considered by SE to be cheating, so don't talk about it in-game, but there are perfectly legitimate uses for it (in my case, I literally cannot play without it. I have a scheduled task running that causes a window to pop up every 7 minutes, which will crash FFXI since it needs to be in full screen mode.) It's far from perfect, and it makes the game almost impossible to play if you resize the window at all, but it can be very helpful. Other than that, I can't think of any. As Bomber said, you can macro just about any action in the game, including battle commands, dialogue, interaction with npcs and inanimate objects... Think of it like the command console for Windows.