Wolfram Research
Wolfram Math World: Significant Digits
This site defines and describes significant digits along with examples. Includes references, citations, and links to further information.
Other
Easyphysics: Chapter 1 Basic Skills
The resource examines the basic math skills needed for studying physics. Some topics include units, scientific notation, significant digits, and trigonometry. Example problems, graphs, and a chapter quiz are included.
Math Is Fun
Math Is Fun: Rounding Numbers
What is rounding, and what does it mean to round up and round down? Learn how to round numbers via simple steps and examples.