On top of the already mentioned:
- http://www.lugnet.com/
- http://lego.wikia.com/wiki/LEGO_Wiki
- http://www.brickset.com/
- http://www.peeron.com - Large set database and instruction scan library
- http://www.bricklink.com - Lego marketplace selling millions of used and new parts and sets. This is also a nice reference for information and inventories related to old sets.
I personnally use the following ones as well:
- http://www.brickfactory.info/ - Online collection of Lego Sets instructions
- http://rebrickable.com/ - Site allowing you to see what other Lego set you can build from your existing collection
After that, it all comecomes down to forums and more specialized websites that directly depend on your area of interest (sci-fi, military, castle...), your language, and your location.