You're familiar with the haiku, right? A short poem where the lines have specific numbers of syllables (5, 7, and 5)?
So let's presume that you're playing through Opposing Force. You bring up the Console by hitting the "`" key (it's the same key as the "~", but without the Shift). Then you type the word "haiku" (without quotes, of course). Hit Esc to be taken back to the game and be treated to a short poem that probably won't make much sense
And you can repeat for more
You have to read fast, though, because they fade out very quickly.