Even for criminals...

Did you sell a few kilos of marijuana, take a bribe or steal a computer in 2006? Be sure to include that income on your tax return, unless you want to get in trouble with the Internal Revenue Service.
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Bribes. If you receive a bribe, include it in your income.
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Found property. If you find and keep property that does not belong to you that has been lost or abandoned (treasure-trove), it is taxable to you at its fair market value in the first year it is your undisputed possession.
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Illegal income. Illegal income, 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.
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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.

Read the whole thing for even more income you might not know you need to report.  Did you receive a gift as a hostess to a candle party?  You owe extra.  Receive a reward for something last year?  Better pay attention.  Via Hit and Run.