[Sekrin]

• Cowtip is the most customisable one I know of (http://files.wowace.com)
• Engravings lets you have places you might find items appear on the tooltip, Mendeleev also puts extra data there (although not locations I believe) and Sellfish gives you vendor prices for things (if you get Sellfish I also recommend Vendorbait as it highlights which quest item is worth the most money - very useful! All of these are available here.
• Quartz is the most customisable casting bar I know of (files.wowace.com again). Having said that, a lot of UnitFrames addons come with builtin castingbars these days.