I currently use chronos ( a huge package of stuff that has a lot of really nice uses and others are just pointless to me, but you can turn off the ones you don't need or want), KTM threatmeter, natur enemycastbar, and CT_Raid. I use about 4-6MB of memory for all the add-ons i use but a lot of them i've used since I started playing and don't know how to play without them, or don't want to think about trying.
All of them can be found at cursegaming.com, or you could just google "wow mods" and every site that has any will be on the list.