Chemistry software I can strongly recommend Chemland 6.0 and The irYdium Project Virtual Laboratory, but this site has links to many other goodies.
More chemistry simulators This lot you can run online so you don't have to clog up your hardrive. The site is very basic and is just a list of simulators with links
Even more chemistry simulators from Iowa State University. Very good!
Possibly the best visual aid simulator for physics and chemistry
pH and hydrogen ion concentration
Interesting article on the need for proper education in chemistry
Physics simulator interactive software This site also includes some math software (quadratic equations and curve fitting)
Oscilloscope for Windows Turns your soundcard into an oscilloscope. You have to pay for this one, but you can use it free for two weeks.
If you want to download and use some of my simulators, be aware that some are written in Excel 2003 and some in OpenOffice a free product similar to Microsoft Office 2003 and it is FREE and in my opinion, better.
Stellarium is an astronomy simulator. Choose your location on Earth, The Moon, Mars, et c.; choose the timebase; see the constellations; identify stars, nebulae et c.; switch azimuth and equatorial grids on and off and have fun.
More FREE stuff:
I use Melody Assistant to make and edit MIDI files and convert them to mp3 files, so my students have sound files with which to practise.
For recording live or for editing existing music files (to make loops for practise purposes), I use Audacity. If you want to convert your Audacity files to mp3, you will also need to download LAME mp3 Encoder, which is on the same page.
As an interesting visual backdrop to my live performances, I play my mp3 backing tracks using Winamp. This has the best music visualization software I have found; I project this visualization as a stage backdrop.
You don't have to write MIDI files from scratch, because there are many free MIDI file sites like Midi Mole