Pitbull, AG Unit Frames, or Perl / X-Perl are the big name unit frame mods. I prefer AG or Pitbull, but X-Perl is a very good mod to start with. Once you understand the mod check out the others.

As far as low memory usage goes, AG is less than pit. I don't know about X-Perl.

Pitbull has 1 feature that AG does not have though, and that is mouse over unit frames. Which for multi boxing might be good. It will show the unit frame of who/what ever your mouse is pointing at, and it will also show their target. Plus you can use mouse over macros, to cast on the person your mouse is over.