Presentation
It's not easy to talk about science when you get back home after a whole day doing nothing but physics, chemistry or biology. This section might therefore be filled rather slowly. I'll start by avoiding the question: I'll post a few links to some sites that I use or like and that may be handy for some of you.
Websites of popular science / initiation
Chaos-Math
This this, though it deals with physics, has been designed by a community of scientists, including people from the maths department of the ENS. It is a very well-explained introduction to the notion of chaos; to give you an idea, this is (among a lot of other things) what makes the weather so difficult to predict over long periods of time (typically more than three or four days). I think anyone can enjoy it because it presents a lot of things, from the most basic notions to others that are much more advanced, but always in a very educational way.
Dimension-Math
Another website made by the same community, where notions related to dimension are introduced: geolocation, stereographic projections (maps), 4th dimension, complex numbers... They really made a great work, explaining things that are all but simple in a way that makes it much more intuitive.
Websites on chemistry
ChemSpider
This is THE site where you'll find (almost) all there is to know about any compound you may use. You can find there basic properties like the average mass, the characteristic temperatures, or the density, as well as more specific data such as IR and UV-visible spectra, etc.
Organic WorldWide
A useful site for organic chemists, with notably a lot of bibliography and the basic methods regarding synthesis, experimental setups, spectroscopic tables...
SDBS
This is the most comprehensive website I know when it comes to the spectra of chemical compounds. You'll find there a large number of reference spectra, be it in IR, NMR, EPR or Raman.
Webelements
Everything is in the title: you'll find all about the periodic table on this site.
Nice websites on science
SciStyle
This site, mostly related to biology, will allow you to see a more artistic approach to science (which is too rare, to my mind). You can also learn the basics about the PyMol software on this site.
BioVisions
Still on biological subjects, and in collaboration with Harvard university, this page displays beautiful videos of the processes that take place on the cellular scale. I think it makes learning about cells very enjoyable.
FoldIt
As it seems that every "fun" scientific website I know is about biology, let's keep going with FoldIt, a game created by scientists and presented as a collection of 3D puzzles. Your objective shall be to find the best configuration of a protein thanks to different tools. The game is very intuitive and has a very good explanatory introduction. I'm sure puzzle addicts will love it.
10K apart
Though it is not really about science, I have to give a link to this wonderful site. You will find there a lot off small web apps, some of which are related to science, like L-Systems, Mandelbrotter, GraviScript, Life!, Fractal Discovery, Turtle Shell, Graphr, Graph 10K, Spiro10k, Attractor, Magnetic. Others are about almost anything and there are quite a few games.
