Wizards of the Coast offers a wide selection of Judge Articles that deal with nearly everything a Judge should know. If you are new to this site make sure to also have a look at the older articles section, where you can find articles that may have been written years ago, but still carry a lot of information.
If you have an interesting topic you want to talk about, you can submit your own article. Visit the Article Submission Website to find out what you have to do.
While the official articles tend to focus on policy and tournament organization, there are a lot of other sites that give answers to rules questions but also deal with policy from time to time.
Most Famous is "Ask the Judge" on Starcity Games. On fridays there is "Feature Friday" which gives more information about judging in general. See here for a list of all Ask the Judge (and its predecessor Final Judgement) and Feature Friday articles.
Cranial Insertion is a weekly column where rules and policy questions are answered with several translations.
German: PlanetMTG had a regular column scheduled on Fridays that ran until November 2008. It offered a wide variation of articles from rules questions to information about tournaments or the Infraction Procedure Guide.