Shadowgreenlight is a nice addon that tells you the perfect time to cast Shadow Word Pain, and recast if needed.

It realy helps your dps.

http://wow.curse.com/downloads/wow-addons/details/shadowgreenlight.aspx

Also a nice one (but I guess most already have this) is DoTimer
With this addon you can keep track of all your dots and cooldowns. This way you can make sure your have a max uptime of your dots.

Also a great way to up your dps

http://wow.curse.com/downloads/wow-addons/details/do-timer.aspx