Titan plugin, that shows which zones and instances fit best for your level. You can hold your mouse over each zone, to see what level that zone is recommended for.
--!! I AM NOT THE AUTHOR OF THIS ADDON !!--
The original author from what I've updated is Utruk.
Bhaldie has made an AddOn similar to this too.
I just updated it, since it was totally messed up, and it didn't work with TBC.
Don't have much German, French or Spanish localization yet... I would like some help with that.
