Rich NRICH Shares the Wealth
Monthly editions of NRICH provide both students and teachers with a myriad of interesting content (problems, games, articles) that both challenge and "recreate" one's thinking process in mathematics. The content on the NRICH website is created by a team of teachers who are also "practitioners in RICH mathematical thinking." The host is the University of Cambridge.
A different theme umbrella's the content each month. Consider these past themes:
 July 2010: Modelling Scientific Exploration and Discovery
 June 2010: Mathematics and Art
 May 2010: Complex Instruction and Groupworthy tasks
 February 2010: Collaborative Mathematics
 October 2009 & March 2009: Visualising
This is only a sample of themes, as the website has about ten years of "rich" material on it.
The content and activities are separated into Key Stages 1 to 5, roughly equating to age groupings from 5 to 19. All of the resources have been classroomtested...and the website claims they "do make a difference."
A relatively new section is stemNRICH, which offers "mathematicalscience problems devoted to exploring the richness of the vital mathematical ideas underlying science, technology and engineering in thoroughly physical, relevant and engaging contexts." [Try to say that outloud ten times quickly!]
NRICH also offers a "AskaMathematician webservice," where a team of mathematicians will respond to any mathematical questions you (especially students) have. Rather than respond with answers, the team guides one through their own reflection on the problem and its possible solution.
I found it interesting to browse these questionresponse interactions, often observing a good model of how to help students answer their own questions.
