
Tags I've seen used:
SeeD (RuneScape): SeeD or RS
Defenders of Aderon (Guild Wars -- Kurzick): DoA
Legion of Aderon (EverQuest II -- Oasis): LoA
The Eranara Construction Company (Blockland): ECC
Knights of Arcturus (Age of Empires series): SKoA
Aftermath (GunZ): GunZ
The Light Side (Day of Fools events): TLS
The Dark Side (Day of Fools events): TDS
HighLANder Clan (Real Life; also Day of Fools events): hLd
Order of the Black Fedora (A philosophy of sorts): OBF
Xbox Live: XBL
Factions that don't seem to have tags:
Gunblade (MapleStory)
Guardians of Arcana (World of Warcraft -- Eitrigg -- Alliance)
Defenestrators (World of Warcraft -- Kalecgos -- Horde)
Forgotten Order of Taris (Star Wars Galaxies -- Intrepid -- Rebel & Neutral)
Knights of Aderon (Diablo II)
GunBlade (Kingdom of Loathing)
Defenestratorz (Yahoo! Games)
Manetheren Warriors (StarCraft)
Orange Star Aces (Flight Simulators genre)
SeeD (Medal of Honor: Allied Assault)
You can probably tell I'm bored.
