Tom Igoe's physical computing website:
www.tigoe.net/pcomp/

Dan O'Sullivan's physical computing website:
itp.nyu.edu/~dbo3/physical/physical.html

Basic Stamp 2:

Tom Igoe on the Basic Stamp 2:
http://www.tigoe.net/pcomp/stamp/index.shtml

Parallax, who make the Basic Stamp:
www.parallax.com

Murat N Konar's Basic Stamp programming application:
www.muratnkonar.com/otherstuff/macbs2/

Electricity and electronics:

Work out the value of your stray resistors with this handy graphical tool:
www.dannyg.com/examples/res2/resistor.htm

Find schematics and other info for an electronic component:
www.findchips.com