I've released my first World of Warcraft AddOn. It monitors XP as you receive it, displaying how many more rewards of that amount you'd need to earn to gain another level. It also adds a permentant display of your current progress to the right of the XP bar, indicating your percentage complete of the current level, and your current TNL.
Unzip the contents in your World of Warcraft install directory to install the addon. It should work in both the PC and Mac versions of the game.
Updated 2005-02-15: Bumped up the Interface version to 4211, otherwise it's the same.
Edit 2005-05-03: Due to site rearrangements, versions prior to 4216 are no longer available. They're all the same, anyway, minus the Interface version marking in the TOC file.