Here is a few websites I have found on my travels around the web. I will keep updating this as I find more but feel free to send me a message if you want me to add any links you have found useful or interesting. Stamp related links only guys!  

The Royal Philatelic Society London
http://http://www.rpsl.org.uk/about.html

"The principal objects of the Society are:

1. To promote, encourage and contribute to the advancement of the science and practice of philately.
2. To inform members of the Society on all matters affecting any of its objects by meetings, discussions, displays, lectures, correspondence or otherwise; to assist and carry out philatelic research, and to print, publish and issue such papers, periodicals, books, circulars or other literary matters in support of these objects.
3. To hold, either alone or jointly with others, promote, or subscribe, or assist with international or other philatelic exhibitions in the UK or elsewhere, and to offer and award prizes, medals or other recognition in connection with such exhibitions or for any literary work connected with philately.
4. To establish and maintain a library and collections of stamps, designs, proofs, essays and other articles of interest relating to any of the objects of the Society. "

The Siegel Encyclopedia
Informative American site with articles about stamps, when and why they were issued.
http://http://siegelauctions.com/enc/enc.htm

Stamp Forgery Guide
Part of a website called Philatelic Experts, this guide may or may not prove useful to some. I can't guarantee all the information on the site but its a useful starting point.
http://http://www.filatelia.fi/forglinks/index.html

http://http://www.stamp-links.com
The best stamp collecting websites on the internet