For Windows pIRCh is considered by most to be the best, most user-friendly and most adaptable. Download it from Koach's pIRCh site and you can get Koach's installation kit to make it even easier. For years, Koach has been the one training MSN chat hosts and sysops, and his Web site has a wealth of information for beginners and advanced alike. But all you have to do is follow the instructions on the download screen and you'll be up and running. When he tells you to type in "chat.msn.com" put in "chat.annexcafe.com" instead. When he tells you to put #KoachsWorkShop, put in #MansHeart instead. That's all there is to it!
For Windows 3.1 and Windows for Workgroups mIRC has a 16 bit program that will work for Windows
3.1 and Windows for Workgroups (which I seem to recall is Windows 3.11). You
can download it from their Web site, but just be sure you get the 16 bit
version, not the 32 bit one.
For the MAC You can get Ircle (which is shareware)
at their Web site in a fully-working try-before-buy version.
The Tucows Web site gives Snak its highest, 5-cow rating. You can download it from their Web site.
For UNIX systems Check out UNIX
If just want more info on IRC (the Internet standard for chats on the Web) and how chat clients use it, MSN has a nice FAQ. Or you may want to visit the IRC Help Web site for lots of very helpful information, especially the FAQs.