Irssi is a modular text mode chat client. It comes with IRC support built in.
Irssi is completely themeable. Every single message can be themed. True colours can be used in your themes. Check the theme gallery for some impressions.
Irssi makes it easy to write protocol modules in C. You can enjoy the chatting power of Irssi in combination with IRC, ICB or SILC. Find more modules on the Modules page.
Irssi features a sane and minimal scripting API together with a coherent signal handling mechanism. There are many existing scripts for Irssi, see Finding scripts for some good resources.