http://www.gleasonresearch.com/ sells Handy Boards and Handy Crickets, Crickets accessories and the software to control some wonderful scaled-down robots (from Fred Martin and Wanda Gleason).

http://www.techno-stuff.com/ is a great place to buy other Mindstorms compatible sensors and controls.

http://www.legoeducation.com is the place to buy Mindstorms for Schools kits and replacement parts.

Used and new LEGO replacement parts are sold by the vendors at www.bricklink.com this site is especially good for hard to find colors (although you might have to buy from a vendor outside the USA.)

The Carnegie Mellon Robotics Store sells educational CDs and "camp on a disk" programs.

The POW Science store has robotics kits and other science supplies.

Mr Robot (TM) sells the Talrik, Jr. Robot, Microcontrollers & Kits, Expansion Boards and much more.