What are the most useful shortcuts for performing back-of-the-envelope calculations of computing problems?

Question

With a back-of-the-envelope calculation you can mentally obtain an answer to a question, within an order of magnitude of its actual value. Some shortcuts can simplify these calculations. In computing I've found the rule

10³ⁿ is-close-to 2¹⁰ⁿ

invaluable. For instance, through this I can say that I can binary search through a billion records with about 30 operations. What other shortcuts do you use and recommend? Let's put them all here and vote your favorites.

Answer 1

10³ⁿ is-close-to 2¹⁰ⁿ

Answer 2

Light travels at one foot per nanosecond

Answer 3

pi seconds is a nanocentury

Answer 4

G ~10 m/(s²)
Dimensional Analysis
Rounding (simple, yet effective)