Phillipe “Philo” Hurbain, co-author of Extreme NXT: Extending the LEGO MINDSTORMS NXT to the Next Level (Technology in Action) has a wonderful website dedicated to the technical aspects of LEGO NXT and its component sensors and motors.
Philo’s website has several tabs corresponding to different topic areas:
LEGO® MINDSTORMS™ & LEGO Technic: a variety of interesting projects
NXT: details about LEGO NXT components and NXT-specific projects
Sensors: detailed information about sensor workings and electronics
LEGO Tech Info: LEGO technical information
LDraw: tools for LDraw part authors
LEGO & Photo: accessories for photographers
My favorite pages are:
LEGO® 9V Technic Motors compared characteristics studies the various characteristics of a wide variety of LEGO motors.
http://www.philohome.com/traction/traction.htm studies the capabilities of a wide variety of LEGO wheels.
and check out his new book: