Links to web-forums where you can discuss your LEGO matters
Lowlug Dutch language /Nederlandstalig

Links that lead you to lists of links to other nice LEGO pages:
search with "LEGO" and find more than 30 LEGO webrings at RingWorld
Cool LEGO Site of The Week    
LEGO Maniacs' Guide

German/Deutsch: 1000 steine
The Legoland Times

Links to software for computer aided LEGO design:
Ldraw  MLcad   for drawing LEGO-models.
For this site I used a model (M6596A) from: the L-Draw Model Repository
3DWin   is a program for coversion between 3D-formats and is capable of converting the Ldraw-format!

Links to pages that appeared to be usefull for me:
Doug's LEGO Technic Tri-Star Wheel ATV and Robotics page   
Lego Technic and Tri-star wheels
LEGO on my mind  
Denis Cousineau Technic Lego Page   
CSSOH’s LEGO Pneumatics Page

Find scans of all the original building instruction you need at
LEGO® Instruction and Catalog Scans

Challenges I haven't dealt with yet.
Cars with automated gearing. The idea is to use the resisting force that slows down the car or even blocks the motor for switching gears. A very nice demonstration model is constructed by Jeff Jahr
Continuous variable transmission. Explanations and one solution is given by Rob Stehlik
Omnidirectional wheel. Mark Warren pointed me to Acroname's omnidirectional wheel. Their Palm Pilot Robot Kit (PPRK) uses these wheels as part of a Killough platform. This might provide another way to create wheels for Lego® holonomic platforms.

Links to very special LEGO-pages
Look closely to this picture from the famous Escher and try to think of a way to build this in LEGO. Andrew Lipson found a way to do it in: Escher's "ascending and descending" in LEGO

One of the most controversial sites: Blockdeath: a museum of horrors

Very beautifull: The Minifigs museum of modern art

Fun with CD's