I use Mumble, a very light weight, no in-game setup, chat mod. I miss a few features, but it's light and it's staying, unless someone can point me to a similar light mod ;) I miss eg classcoloring. But I think Teknicolor does that? Mik's Scrolling Battle Text
Align, form WoWi, it's been mentioned.
I used to swear by Satrina's Buff Frames, but replaced them with - depending on my mood - Elk's Buffbars or Buffalo2. Satrina's mod Chat Frame Extender 2 is quite good as well, it's next to mumble my perfect chat mod.
@Jesamyn: LOL@ your personal message... Well, if you're not looking for one, then I can't help you...
I couldn't resist. I thought maybe yours was a response to my 'not the droid you are looking for' (from the Star Wars line, of course), so I had to see if you'd notice if I changed mine. ;)
Hrm, and lemme see...for more on-topic-ness, I also like Battle Cry. It's for quotes on events, with a cooldown and percentage on each event. I know there are newer mods that probably do the same thing, but since it works and doesn't use hunka-memory, I'm just sticking with it.
PvP Cooldown - a nice little movable frame for those of us not using a standard player frame (in my case, IceHUD), who won't see their PvP flag drop. HonorFu doesn't count down correctly, and I don't have a need for all fo the stuff in BattlegroundFu (dunno if it has a countdown, either). This works, so I use it happily, though one day when I am really bored, I fully intend to change the border.
*looks around* I don't have enough coffee yet. I blame Indy.