For the low, low price of five dollars, a user can activate a word filter, called an emote, that will make a small GIF appear on /b/. It works by placing your choice of text inside two colons (:cockbean:, :eng101:) and then having that text filtered to the GIF of the user's choosing. Popular examples include Professor Smiley, Ice Burn, and Pwned. Emotes are inspired by/borrowed from the SomethingAwful forums.
The most well known of these emotes was :zdx4875: which activated Professor Smiley. It was made public on /b/ in the Winter of 2005, and for about 24 hours before the mods de-activated it, /b/ was filled with the Professor.