Overachiever is an addon that tries to help you complete achievements. It does this by adding helpful notes to tooltips and by adding a couple of tabs to your achievements window.
In the picture I have moused-over the Glassfin Minnow School on the map, and Overachiever has added the note that I need to fish in that pool for some achievement.
Opening up the Achievement UI (Y) shows that Overachiever has added two tabs, “Suggestion” and “Watch”.
The “suggestion” tab appears to be based on your location and suggests achievements that are important to your location. So while flying over Grizzly Hills it tells me I haven’t completed the PVP achievements there. I’ve found this -VERY- useful while running serial Random Dungeons. Popping open this tab quickly and easily tells me which acievements I havn’t completed in -this- dungeon.
The “watch” tab allows you to ALT+clicking any achievement to “watch” it. This makes it easier to find again rather than having to dig through all the other menus and tabs.