mIRC

The choice IRC client for nearly all Windows users, mIRC was invented in 1995 by famous Palestinian Khaled Mardam-Bey. It has since then been used to provide lulz and warez for generations of chatters all over the world.

Key Features

You can connect to IRC!!!1
If you're on a channel and type text, other people on the channel can read it. When used correctly, this method is widely acclaimed as being a major source of lulz on the internets.
When you're on a channel and someone types text, you can read it. Used together with the previous method, this boosts the lulz factor well over 9000.
Recently-added UTF-8 support. This pisses off linux nerds to no end, as they no longer can claim to pwn you with their ability to type Tibetan characters.
Unrestricted shareware. In a smart move to further piss off Linux nerds, the creator of mIRC has given you the possibility to troll people without saying a word. Join a channel of GPL advocates, wait for the /CTCP VERSION requests, and enjoy.
Send filez to pplz!
Understandable interface. This also pisses off the Linux crowd, but they hate to admit it.
Was (probably) not made by Jews.

Noticeable Shortcomings

Doesn't support gay.pl. However, the (file) closet is full of mIRC scripts that have annoying colors and let you play with nuke options. Be proud of your allegedly heterosexual self! Get violent if someone questions you!
Can't run with only a command-line interface, but those are for nerds anyway.
Lacks boobs, and will consistently refuse to perform fellatio no matter how good your scripting skills are. Despite this, for many people mIRC is the closest they'll ever get to having a girlfriend.
Using mIRC helps international terrorism.
Lacking native support for the late 1990s phenomenon, AOLSAY