Official X-Chat for Windows version has been made shareware (30 days trial), but X-Chat is still open-source and licensed under GPL. So there are a few free non-official builds, and mine is among them. You can find more in Wikipedia.

X-Chat has two good features of usability: colored nicknames and alignment by text as opposed to alignment by nicks. When you read text, it's enough for eyes to notice the colors of nicknames, which they can do when they are focused to somewhere else near, and you may know at once who is writing. Colors are inobtrusive and there just 6 of them—not to burden your short-term memory. And eyes don't have to search for ends of nicknames, they can start reading text at once when text is aligned by the beginning.

This X-Chat for Windows version features:

• 41 language translations;
• encrypted connections using OpenSSL 0.9.8e;
• native IPv6 support on Windows with IPv6 stack;
• Perl, Ruby and Tcl language interface plugins for scripting;
• Winamp, DNS and EXEC binary plugins;
• various themes with graphical theme and font utility.
• It is compiled using Microsoft Visual Studio .NET 2003 Architect Edition on Windows XP SP2. To enable language interface plugins, you need to install respective language distributions (see Download page).