Since icyveins and elitistjerks have already been recommended as sites, I'll recommend another addon. OmniCC. It's a very simple addon but I find it really useful. When you use an ability a number will show up on the icon, letting you know when you can use that ability again. When there's only a few seconds left it will count down in red so it's easy to spot when you can press it again. You can also use the site askmrrobot.com, it helps you find new upgrades and lets you know how to gem and reforge your gear for 100% optimization.