David Lynch

writes World of Warcraft addons
AuldLangSyne
Massively improves the friends list
FuBar_QuestsFu
The quest log: a complete replacement, with many improved features
FuBar_ToFu
Flight time remembering
FuBar_KungFu
An easy way to view your skills
FuBar_PetInFu
An easy way to view pet information (discontinued)
BankStack
Fills stacks in your bank from your bags.
SilverDragon
Tracks the location of rare spawns
PartyToGo
The party quest status display module of QuestsFu, ripped out and made standalone
OpenAll
Sometimes you just want to open all your mail
InBank
How many of that item are in your bank?
SimpleTradeskill
If I visited a vendor and the bank, how many of that recipe could I make?
Quixote-1.0 (lib)
A library for accessing the quest log, and for communicating with other players about it
TheSocial-1.0 (lib)
A library for accessing the friends list

Also contributed to...

Mendeleev
Rewrote the sets menu, added most of the 1-59 dungeon reward sets, jewelcrafting, and lockpicking data
BulkMail2
Wrote the GUI
PeriodicTable-3.0 (lib)
Added lockbox skill requirements
PitBull
Wrote the Portait module
ag_UnitFrames
Wrote portrait system (notice a theme?)
Quartz
Rewrote the flight timer module, and added support for ToFu
ColaLight
Rewrote entire display and tooltip hooking functionality

Ancilliary utilities

wowace.py
A Python-based addon updater for addons hosted on wowace.com.
"Top list" data miner
Blizzard publishes daily rankings of various bits of data -- most killed mobs, deadliest mobs, most completed quests, and so forth. I wrote a parser to grab and save this stuff, intending to write an app to make it more searchable. I never got around to that... but it does work.