Jeff, he of the mightyOhm, asks:

Have you ever wondered why standard 5% resistors have strange values, like 330 and 470 Ohms, instead of nice round numbers like 300 or 500 Ohms?

It turns out that standard resistor values form a preferred number series defined by the EIA.  5% values are part of a standard called E24.  The standard is based on a geometric series – each value is approximately 1.1 times the previous one in the set.

EIA Resistor Values Explained [via The Steampunk Workshop]