Space

NASA SpaceWeather site (Sun/Earth)
Latest MeteoSat image of Britain
Sky & Telescope Online
Astronomy Picture of the Day
Solar Terrestrial Dispatch, a website which has predictions of auroral activity. Download either the SWIM, or Aurora Monitor programs which will gather data from a variety of sources about the sun and aurorae.
A website by astrophotographer Robert Gendler showcasing many beautiful CCD images.
Nik Szymanek who gave us a talk recently, has a website where many of his excellent images are displayed.
Heavens Above, a website which has predictions of visibility of satellites as well as the moon and planets.
Cartes du Ciel, arguably the best free planetarium program.
BBCi's Space Homepage includes the Sky at Night Archive in case you missed it.
Meteor Calendar by Gary Kronk.
Green Witch, a local astronomical supplier. Has a large range of telescopes, accessories and books.

Other Societies and Organisations

The British Astronomical Association
The Royal Astronomical Society
The Institute of Astronomy (Cambridge)
Cambridge Astronomical Association