and the plugins:
It's the "firefox" of IM chat clients. That's not to say Trillian is the IE of chat clients, but with Miranda there are a number of plugins that you can use to get features you are interested in.
For example (and this is a minor one), I have a plug-in that allows me to get my local weather report as a contact (I can't chat with it though).
In addition, it has a smaller RAM footprint, so if that's of concern, it's great.
It supports MSN, YAHOO, AOL, ICQ, Jabber, and some other things.
Supports MSN Avatar support, but as of yet, not the video or "nudges" (which are lame anyway). It sits there right in my launchbar without doing anything flashy.
It does take a little customizing, and you'll need plugins to get a large number of smilies, but you can always go back to the site and see if there is anything new and interesting to install.
I've been using it for about a year and a half now and love it.
EDIT: For those of you who want to use Miranda from Trillian but don't want to lose your history, there is also a plugin in to import your Trillian Histories. I don't know how well it works, as I've never used Trillian for an extended period of time.