Eggdrop is the world's most popular Open Source IRC bot, Designed for flexibility and ease of use, and is freely distributable under the GUN General public lisense(GPL).
Eggdrop was Originally developed by Robert Pointer.
Some features of eggdrops -
Designed to run on Linux, *BSD, SunOS, Windows, MacOS X, Ect....
Extendable with TCL scripts and/ or C modules
Support for the big Five IRC networks (Undernet, DALnet, EFnet, IRCnet, and QuakeNet)
The ability to form botnets and share partylines and userfiles between bots
Some benifites of Eggdrop:
The oldest IRC bot still in active development (Eggdrop was created in 1983)
Established IRC help channels are websites dedicated to eggdrop
countless premade Tcl scripts and C modules
Best of all Its free!!