Cross-platform IRC clients
ChatZilla runs as an addon for Firefox and Seamonkey, or standalone with the help of XULRunner, and as such is portable to all platforms on which Firefox can run.
LeafChat is a graphical, standalone client written in Java, and runs anywhere there’s a suitable Java runtime.
Web Based Clients
Webchat interfaces are listed elsewhere, and are available anywhere there’s a working web browser.
Linux/Unix Clients on Windows
Many of the terminal-based Unix/Linux IRC clients have been ported to run on Windows with the aid of Cygwin or MSYS. These ported clients may be awkward to use if not comfortable with console-mode applications, and primarily cater to advanced users.
The popular X11 client, xchat, also has a Windows version, which is a common alternative to mIRC on Windows systems.
Windows Clients on Linux/Unix/OSX
mIRC is known to work well with Wine.