My UI goes something a little like this
1.Damage Meters
2.Minimap Button Frame (the green strip in the lower right hand corner)
3.Omen Threat Meter
4.Prat 2.0 (this is what I used to customize my chat window)
5.Spartan UI (The main Action area itself including the minimap), Thanks Kitty
6.Tbag (hehe) The main bag unit, opens up all your bags in one
7.WoW Latency (minimap icon on the button frame)
I also have a Tanking addon which warns who has aggro aside from the tanks themselves, but I've yet to test it.
Following Lea's advice I downloaded SpamMeNot, which again is a very useful addon
Others include, Atlas(All), Cartographer, Auctionner(all), Scrolling Combat Text, GroupCalender (EVERYONE should have this by now), Questhelper, y'know the obvious basically, see ya online all =]
