DarkruneDKLevelUp (WotLK)

Here are a quick recap of what the addon contains:

UI to keep track of Xp, rested xp and rep with the ability to enable an option for hunters to track pet xp as well.
Favorite mount feature (ground and flying is seperate) on a character basis.
Quick setup of actionsbars using a command and some settings in the addon.
Dungeon feature, which tells you how much raw gold you made running a dungeon upon completion (exit).
Auto-repair whenever you speak to someone who can repair your gear.

DLU HQ

If you have the addon on retail this might not be a new thing, but the addon has a UI, that helps track various stuff. The things that the UI tracks are leveling related stuff (xp, rested xp, time spent in the current level as well as total playtime), reputation and there is an option to enable some hunter related stuff (tracks pet xp and notifies the player if their pet gets hungry).

Other stuff

The addon also has some options that can be enabled for auto-repair (repairs your equipment automatically then you talk to someone who can repair it). There's a command to easier add all the actionbars instead of having to go into the options and clicking all the checkboxes.

The addon also adds command support for favorite mounts. This is a bit more tricky however since it wasn't originally supported in WotLK, but it is explained a bit in the FAQ in the addon in-game.

If you are looking for the retail version of the addon, it can be found here:

DarkruneDKLevelUp (Retail)