Links, every website *has* to have one page like this :-)
I thought that I would list a few places that I go to regularly on the internet and some I don't go to very often but still contain useful information.
Astronomy and Physics Societies:
University of Birmingham Astronomical Society - I was the Chair of this society!
Federation of Astronomical Societies - A National Organisation of Astronomy Societies, the above is an example and I am their Secretary.
Venustransit.co.uk - A site I designed!
Society for Popular Astronomy - A fantastic website full of useful information
West Midlands Federation of Astronomical Societies
Royal Astronomical Society - I am a fellow of this excellent society
Poynting Physical Society - I run this site so I had to link it really!
Astronomy and Astrophysics
Heavens Above - Great for the amateur astronomer provides daily information on the local of satellites and there passes any location given and provides star charts for any location - great!
Space Telescope Science Institute - Educational resources for astronomers
British National Space Centre - lot's of useful information
Spaceweather.com - excellent site with lots of brilliant resources
University of Birmingham Astrophysics and Space Reasearch Group
University of Birmingham Observatory - a site I designed!
The Cavendish Astrophysics Group - astrophysics group of Cambridge University
The NASA Astrophysics Data System - a wonderful resource
The Extrasolar Planets Encyclopaedia - by Jean Schneider
NASA/IPAC Extragalactic Database (NED) - need to know some extragalactic? NED will tell you.
Sloan Digital Sky Survey - size does matter! This is an absolutely huge database.
SDSS Query / CasJobs - the more advanced way of getting data from SDSS
Photoshop FITS Liberator v2.0 - useful for anyone with Photoshop and who take images in FITS mode
Astro-ph/new - latest mailing from astro-ph, useful for astronomy preprints
Birmingham
beinbirmingham - has some useful info
University of Birmingham - a wonderful place... it is so green.
Physics & Mathematics
Eric Weisstein's World of Science - fantastic reference for astronomy, chemistry, mathematics and physics. I use this quite frequently when I am unsure about any work that I am doing
Nature Magazine - the online version of this scientific journal.
Computing
PC-PALS - A useful reference site and full of fun links.
MSDN SQL - useful list of SQL commands