Feel free to add to this wiki as needed.

Auctioneer -- http://wow.curse.com/downloads/details/78/
Gives vendor prices for items. Also, gives the average for auction prices for items.

Tomtom -- http://www.wowinterface.com/downloads/info7032-TomTom.html
Gives coordinates so that you can easily find quest objectives.

Omen Threat Meter -- http://wow.curse.com/downloads/details/9101/
Shows how much threat you have generated compared to everyone else in the party. A good mod to have if you play in parties a lot and don't like getting killed all the time.