For a healer, SaySapped is a must. It's a simple little addon which says "Sapped" when you get sapped, notifying your teammates around you that you're incapacitated.

I'm also a big fan of Healers Have to Die on all my toons, healers and DPS alike. As a PvP healer I like having the enemy healers pointed out to me quickly so I can CC/Mana Burn them when the pressure on my team is low. When I'm DPSing I'm more focused on the opponents and can spot healers pretty quickly - but I'm far more tunnel visioned on my raid frames when healing, so I know I can use the help.

Macros can also help save your life as a healer. Make sure that you have one to yell for help, identifying yourself as a healer who needs assistance. Addons like HHTD can tell your team that you're under attack, but you shouldn't count on other people's addons for your own survival. Take charge and be a nuisance when you're getting ganked.

I personally use on my holy paladin:

lose control - tells you when and for how long you are incapacited/silenced for, help to work out if you need to trinket to save someone
power auras - track my cooldowns and displays them in a visable spot so I don't have to take my eyes off the field of play
vudoh - healing frames, extremely useful for dispelling and general healing