Spell-Alert - Advises you when your target is casting and what it’s casting. Very important C9 Gold for PvP as you can decide is that whether to interrupt or not, Also useful for boss encounters so that you can time your escape/Cloak of Shadows.
SWStats - A simple damage meter. Good to see how you compare to other members in your group or to monitor your DPS.
KLHThreatMeter - Essential in a raid. It monitors your threat (hate) level against a target. On fights that last 3+ minutes, you want a good control of your hate so that you don’t get hit.
Poison Pouch - Allows you to apply a poison to a weapon in 2 clicks.
External Tools - I personally use a Nostromo N52 in addition to my keyboard. I have a 5 button mouse as well, with Run Speed, Stealth and an /assist macro mapped to the extra 3 buttons. Having everything available without having to move my wrist makes combat miles easier.
I don’t have to fiddle to find the right key to use. I have mapped [TAB] to one button on the Notsromo to more easily select targets and the other 14 buttons are mapped to skills. This isn’t a necessary tool but one that I think is a good investment if you want to do any serious PvP.