HBuffs (standing for Homogenized Buffs) is a lightweight buff/debuff viewer that takes into account Blizzard's concept of buff homogenization. The idea is that you create an "hbuff", i.e. a buff or debuff group, and then you specify which buffs you want to be in the group. You then choose which hbuffs you want to track, and they will all appear in a nice little frame. Whenever one becomes active, it lights up green and you can mouseover to see the person providing the buff/debuff. The addon comes with most raid buffs/debuffs already defined, and it's very easy to change these or make your own.
I've been using this addon myself for the last couple weeks and have been finding it very useful. I always used to get a bit frustrated when I'd do 10k dps in vault one week, and then 8k the next - now it's really easy to see exactly which buffs are causing the change in dps. I also notice immediately when we run out of range of totems, which is great for fights with lots of movement like Lich King.