Have I Done That? adds a "What now?" tab to the achievement frame with suggestions, mostly based on your zone. It'll also add info to tooltips for all kinds of things you need to eat/drink/kill/love/equip for achievements. It does have
one annoying bug, but it's easily fixed by copying and pasting the code from my comment on the ticket.
Zone Achievement Tracker will make sure the "Complete X quests in this zone" and "Complete these quest lines in this zone" achievements are always up in your objectives tracker.
Grail will help you find quests you missed while leveling.
I've tried Overacheiver in the past, and while it did have some nice features, it felt really bulky and never quite worked right, and I don't really miss it. Based on a quick look at the download page, it looks like it's no longer being maintained, too.