Currently I use
Nettalk. Used XChat previously but the modded version I used earlier went kaputt...
very nifty for what I use it for.
The advantages are it's entirely open-source, support for multiple European languages (Dutch, Chinese, German, English, Spanish, Russian, and Hungarian), support for utf, supports large filesizes for dcc (downloading a +13GB file, with several interruptions? No problem. (even xchat had issues with files large as that...).)
There are disadvantages too though, the major one being no support for SSL (though I haven't tried so perhaps that info is old...)
If too many channels arem open the gui is a bit cluttered with those, making it a bit hard to find the right one (an optional treeview would be nice, or automatic sorting option).
There's no port for linux, or any other than windows. (Well, it's open source so someone might make one.)
It doesn't seem to have a limit on number of open channels (unlike xchat), but if too many along with too many dcc running, it might behave strangely, crash included...
But as said, for what I use it for, it's perfect...