Pegasus Mail
A genorus New Zealand programmer has created a wondeful email program. It has all the features of the expensive programs such as Eudora and a few things they lack: for it has the most powerful attachment and filetr ability of all mail clients. You can extend Pegasus Mail with plug-ins, to make it even more powerful. It is so wonderful this program is free. I just wish it went open source so a X11 port would be around.
Platform: Windows 9x/NT, Mac System 7+, DOS 3.x+

Eudora Mail
The popular commercial email client has a freeware counterpart. It is not as powerful as its $49.99 big brother but its free. It lacks a spell checker, clean interface or pwerful filtering. You can however get plug-ins that take care of some of these problems.
Platform: Windows 9x/NT, Mac System 7+

Mail Express
Not to be confussed with Outlook Express, Mail Express is Cybercreek's fre email application for Windows 9x/NT that will remain free forever. It has a spell checker among other features and an American and British dictionary.
Platform: Windows

Safe-Net Mail
A limited email client that filters out all the items deemed "unmoral" in today's society from your children's email. Because your computer is not a sentient being with morality, it doesn't know the diffrence between a book report on the shooting of John F. Kennnedy between an email about killing innocnet animals with a BB gun. In my opinion, instead of using programs which violate First Ammendment rights, you should be installing morals into your children. They should know the differenmce between right and wrong internally, not what a moron string of binarys tells them.
Platform: Windows 9x/NT

mIRC
It has to be the most popular chat client ever. It was created back in 1999 and has quite a following. Version 4 or so is the last freeware version, and it unlike the next versions to come lacks the ability to type in color. The mIRC scripting language is most common so that makes this an IRC client you should consider.
Platform: Windows

Xircon
Mant Linux users who also have WIndows love Xircon. It is similar to many UNIX chat clients, Zircon namely, in scripting and interface. I still feel that Xircon can be improved and many things that I need cannot be found.
Platform: Windows 9x/NT

MS Comic Chat
This chat client is Weird! I mean that too. Instead of plain old chat, it's done with comic strips. You can log onto any server, but it is most supported at Microsoft's server. Very bizarre and difficult for those who are new to chat. It features an IE 2.0 like interface, not to slick but it saves memory which is always good.
Platform: Windows, Mac

Global Chat
This is a fairly simple program which allows users to give each other small files like text files. It is the standard in most internet providers for the ease of use it has. It is integrated into your browser so there is no proxy setup necessary and is just plain simple.
Platform: Windows, Mac

Felix
The first and only IRC client for BeOS. It is graphical like mIRC and Visual IRC, its Windows counterparts. Keep in mind Felix is still in Beta, but it has a bright fuiture for BeOS, both Intel and PPC versions.
Platform: BeOS (Intel,PPC)

IRC II
Way back when windows was just a shortcut tool for DOS rather than a true OS, IRC II was the best IRC program around. A local freenet uses this UNIX shell IRC program and I find it good. it is not graphical, but it can handle all of the features like /msg and /list but no pop-ups or scripts here.
Platform: UNIX, DOS

Ircle
The most popular Mac IRC program because of all the Mac clients, it is one of the few that is not in Alpha test or even works. It is graphical like vIRC, Pirch and so forth. Some dislike its cluttered look and want something simpler but I can't find any other client yet that meet its features.
Platform: Mac