Found items, stolen items, and illegally made profit. I thought this was rather humorous. Check it out if you don't believe me: Publication 525 (2008), Taxable and Nontaxable Income

Illegal activities. Income from illegal activities, such as money from dealing illegal drugs, must be included in your income on Form 1040, line 21, or on Schedule C or Schedule C-EZ (Form 1040) if from your self-employment activity.

Kickbacks. You must include kickbacks, side commissions, push money, or similar payments you receive in your income on Form 1040, line 21, or on Schedule C or Schedule C-EZ (Form 1040) if from your self-employment activity.

Stolen property. If you steal property, you must report its fair market value in your income in the year you steal it unless in the same year, you return it to its rightful owner.
Personally, were I to steal something, I don't think I'd be reporting it. That's just me though.