Since we are curious souls, the QuakeNet staff made a network wide CTCP version request at non-peak hour in the beginning of November 2002 that hit all connected clients at that time (don't try this at home, folks :p)
We sorted the results and present them below, for your viewing pleasure.
clients%client
52 984
63,39%
mIRC - older versions
16 344
19,55%
mIRC - v6.03
1 520
1,82%
irssi
940
1,12%
Trillian
781
0,93%
xchat
45
0,05%
BitchX
39
0,05%
ircII
18
0,02%
epic
3
0,00%
LiCe
10 907
13,05%
others
83 581 100% total
Most people seem to run a Microsoft Operating System (ie: Win95/98/ME/2000/XP), those not are listed here.
NB: Versions of mIRC prior to v6.03 may be vulnerable to security exploits, see http://worms.help.quakenet.org/mirc.php
UPDATE: An expanded list of other clients/bots and various scripts.
James[UH] said on Saturday 28 December 2002
Fehlerzeile11 said on Saturday 28 December 2002
Son-Y|Lucky|LkH said on Monday 30 December 2002
[KBA]SillyG said on Monday 30 December 2002
[XPd]Hydra said on Tuesday 31 December 2002
Conrad said on Tuesday 31 December 2002
HyBreeD said on Wednesday 01 January 2003
Lil_Evil said on Friday 03 January 2003
JM said on Saturday 04 January 2003
Cryo said on Sunday 05 January 2003
Zeitkind said on Monday 06 January 2003
Shell^^ha said on Tuesday 07 January 2003
|BTF|PleXzel said on Tuesday 07 January 2003
Win_a_2 said on Tuesday 07 January 2003
Don^Salem said on Thursday 09 January 2003
port80 said on Thursday 09 January 2003
Igor[Rock] said on Sunday 12 January 2003
OstKaka[TA] said on Tuesday 14 January 2003
GuntherDW said on Saturday 18 January 2003
GuntherDW said on Saturday 18 January 2003
Unda^Presha said on Tuesday 21 January 2003
Thalinas said on Thursday 10 July 2003