List of Helpful Websites for Beginning Collectors
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 comes down to forums and more specialized websites that directly depend on your area of interest (sci-fi, military, castle...), your language, and your location.