Also see From AT to BTX: Motherboard Form Factors  in Webopedia's "Did You Know...?" section.

ATX motherboard specifications
Offers an overview of Release 2.01 of the ATX specification (February 1997) from Intel.

Motherboard form factors
Describes the various form factors for PC motherboards, with separate sections on the Baby-AT, ATX, and LPX form factors. This page is from "The PC Guide".

System Case Form Factors
Describes the different form factors used for PC cases, with special attention to the "Baby AT" and ATX form factors. This page is part of "The PC Guide".

Yahoo!'s ATX form factor page
Yahoo!'s directory of ATX form factor.

Hardware Central
Detailed test reports on the latest desktop and laptop computers. Plus news on trend-setting products just coming into the pipeline.

Illustrated Pentium II and Celeron Building Guide
Contains over 50 step-by-step photographs of assembling an ATX system with a Pentium II or Celeron processor. The site also include a large text section titled "Catching up with the New Technology" for those new to ATX.

Webopedia Did You Know - From AT to BTX: Motherboard Form Factors
You've probably heard the term motherboard a thousand times, but do you know what it really means and how it relates to the rest of your computer?