What are your personal "bicycles of the mind" or "thinking rocks" that you rely on to think or actuate some kind of valuable output?

The George Marsaglia mental pseudorandom number generator method:

• Think of a 2-digit 'seed' number.
• Multiply the second digit by 6 and add it to the first number.
• Repeat once.
• Use the right-most digit of the new number as your random number.

If you chart the results using a spreadsheet you can see a weak pattern emerge but it's not easily human-predictable, which is what makes it useful to me.