Just thought I’d comment on my ‘Other’ vote.
Miranda IM - http://www.miranda-im.org/
Aha. There are many clients in that “Other” group. And e.g. at work we use Exodus mostly. But i have voted Spark, because i use it at my workplace and at home. If i could choose more than one, that would be: Psi, Exodus, Spark.
others: JAJC, neos, Trillian, JBother
chuckle I wonder if I should expand it? I mean people have already voted so probably not. But good to know regardless! Thanks for the comments about what “Other” is for y’all! =)
So it’s pretty close, for me, but my default client is iChat.
I also use Psi and AdiumX regularly, and sometimes Spark, Gaim/Pidgin, Exodus, and Pandion.
Approximately 25% of my 820 users use Miranda.
Myself is using Psi, the one and only. 
I currently use Gajim the most, I used to use Trillian’s Jabber support. (now I just use transports)
After that: Spark, then Psi
No mobile clients, just “other” 
I rarely and mean rarely use a pc, I’m always on my BlackBerry with IM+.
All my 10.4.x Mac’s run iChat, older versions run AdiumX. 90% of PC’s run GAIM, other client is Spark. We are moving them over to Spark.
Grant.
I voted Spark but I am still looking forward to other enhancement. We use the system at work and we need complete history. Spark is good but it pains me that broadcast message history is omitted. I tried Psi and Miranda… etc. They do include group msg in history.
Our existing ancient IM system does have numerous problems, however, failing to see all messages in history really dismays all users.
We have started using Psi and Gaim but now about 80% uses Pandion. Psi is still used and Spark, Pidgin as well.
I am using Pidgin. I like its many features.
Psi is the most common here because it’s the best I’ve found for both Linux and Windows. Xabber is my preferred mobile client, at least for now. Used to like IM+ on Symbian phones.