Here are some chatting services that I use.

Aim - Aol Instant Messanger, allows you to chat with people on AOL for free.

ICQ - A really nice chatting service that allows you to send files through it, and is Quake 2 friendly.  Sort of like a mini-emailing service because it takes messages if a user is offline.  Here are some tools for ICQ.

mIRC - Here is a pretty nice chat service.   If you have seen JAVA chats on webpages, they were based off this.