Here are some of the places on the web at which you can find useful material about Mathematica and related matters.
• http://www.wolfram.com – The main Wolfram Research site. This is a very large site with lots of information buried away. You can also buy the software here.
• http://www.polyhedron.co.uk – A convenient place to purchase the software in the UK.
• news://comp.soft-sys.math.mathematica – A news group dedicated to Mathematica. If you post a problem here, you are very likely to get a solution from someone at WRI, or from another Mathematica user.
• http://wwwvis.informatik.uni-stuttgart.de/~kraus/LiveGraphics3D/ – The home page of "Live Graphics 3D" – a free 'must have' application for anyone involved in 3-D geometry or plotting. It displays 3-dimentional Mathematica objects in OpenGL so that they can be rotated in real-time. To achieve speed, the shading has to be fairly crude, but it also has an option to generate PovRay output (ready for ray tracing).
• http://www.verbeia.com/mathematica/tips/tip_index.html – A source of more Mathathematica tips and Ted Ersek's Supplementary Help browser.
• http://home.earthlink.net/~djmp/Mathematica.html – David Park's Mathematica site, with lots of Mathematica applications in physics.
• http://mathworld.wolfram.com/ – A free maths encyclopedia run by Eric Weisstein at Wolfram Research. This is an absolute treasure trove of information, mostly written to be accessible to the largest possible audience. Think of a mathematical term that you have never quite understood, click on this link, and all will (probably) be revealed!
• http://functions.wolfram.com – An extremely comprehensive set of information on a wide range of mathematical functions, with graphics and many, many formulae. Again, this site is available for anyone to browse.