I look at my list and I don't even come close to 18.
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EQ
AO
DAoC
Horizons
SWG (Beta & Retail)
CoH (Beta only)
WoW (Closed/Open Beta & Soon to be retail)
EQII (Beta only)
MxO (Beta & deleted it after three hours)
Lineage II (Beta Only, won't pay for that crap)
Neocron (Beta)
But if you were to look at the entire list of MMO's including the "technically MMO's" you come up with alot more:
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AO
AC
AC2
CoH
Atriarch
Darkfall
DAoC
DragonRealms
Endless Ages
EQ
EQII
Mankind
FFXI
GW
Horizons
L2
MxO
Neocron
Enropia
Shadowbane
SWG
Wish
Eve Online
UO
And you have MEO (Middle Earth Online) and DnDO (Dungeons & Dragons Online) coming in 2005 as well.