I am particularly guilty of this, embedding non-constructive comments, code poetry and little jokes into most of my projects (although I usually have enough sense to remove anything directly offensive before releasing the code). Here's one I'm particulary fond of, placed far, far down a poorly-designed 'God Object':

/**
* For the brave souls who get this far: You are the chosen ones,
* the valiant knights of programming who toil away, without rest,
* fixing our most awful code. To you, true saviors, kings of men,
* I say this: never gonna give you up, never gonna let you down,
* never gonna run around and desert you. Never gonna make you cry,
* never gonna say goodbye. Never gonna tell a lie and hurt you.
*/
I'M SORRY!!!! I just couldn't help myself.....!

And another, which I'll admit I haven't actually released into the wild, even though I am very tempted to do so in one of my less intuitive classes:

//
// Dear maintainer:
//
// Once you are done trying to 'optimize' this routine,
// and have realized what a terrible mistake that was,
// please increment the following counter as a warning
// to the next guy:
//
// total_hours_wasted_here = 42
//