mIRC is an Internet Relay Chat (IRC) program for Windows. mIRC is a highly configurable IRC client with all the goodies other clients on UNIX, Macintosh and even on windows offer, combined with a *nice* and clean user interface. mIRC offers full color text lines, DCC File Send and Get capabilities, programmable aliases, a remote commands and events handler, place sensitive popup menu"s, a great Switchbar, World Wide Web and sound support, and... a lot more. mIRC is shareware but not crippled in any way...
What is new in version 6.3 [August 16th, 2007]
This version of mIRC offers full Vista compatibility with support for limited user accounts, U3 support, support for tips (those small text balloons that pop up near the Windows tray) to notify you of various events such as incoming messages or users in your notify list coming on or leaving IRC, support for positioning the Treebar at top/bottom of the mIRC window, and a feature that allows you to switch between Normal and Compact modes. Compact mode makes mIRC behave like a messenger with the treebar filling the main window, all windows opening on the desktop, and so on. [full list of Fixes, Changes and Additions in mIRC 6.3]