You're likely running one of two things:
AOSP (Cyanogenmod, Decks, SalvageMod, MIUI) then you want to flash either Tiamat or SavagedZen. My battery life is significantly better with Tiamat 3.3.7 than anything else, but all phones are different so YMMV.
Gingerbread Sense (Android 2.3.x w/ HTC Sense) - You want either the Freedom Kernel or Chop Suey. They're pretty similar, but I get better results with Freedom. There are three different builds and 4 different downloads for each build. Unless you're running the SynergyGodmode rom or a Synergy Nightly build, you want the Universal Freedom kernel. From there, the Aggressive, More, Less and Stock builds refer to how much undervolting the kernel does. Some phones can handle a lot of undervolting, some can't.