Tankpoints is a pretty nice mod for tanks. It calculates your damage reduction and gives you an impression about how much damage you can take on average before you die. This is calculated seperately for melee damage and every school of magic damage. So it allows you to balance your 'tanking stats' in an easy way. Just switch around your gear and see what nets you the most tankpoints or damage reduction (depending on what you're aiming for).
You can download it here: http://www.curse-gaming.com/en/wow/addons-1365-1-tankpoints.html
It's also nice for wannabee prot warriors (aka feral druids) like me :happy: .
(Unbuffed I got more then 73% DR and nearly 20k tankpoints, which means I can take about 20000 unmitigated melee damage before I die :biggrin: ).