Calculators are a necessary tool in science and math classes. But, they don't have to cost the students anything.
First, there are a ton of free calculator apps available for every smartphone (iOS, webOS, Android, BB, Windows) so any student with a smartphone or PDA can get one on there. The paid apps are usually only $0.99 - $1.99 too.
Second, there are some free downloadable calculators, such as Microsoft Math and most computers have a built in calculator.
But, free, online calculators are also a great resource for educators and students. There are calculator extensions for Chrome and Firefox that students can use on their computer or through their Google account.
Here are some free online calculators that I like:
Richard Byrne over at Free Technology for Teachers also recommends EnCalc as a great, free, online scientific calculator.
A Better Calculator is a new, free, online graphing calculator from Demos (who also offers a service to create online interactive media). There is nothing to download as it runs online. It is easy to use, full color, full functioned. You can share your work online or even share it with others (like your students) or students can share it with their teacher.
Google is now a graphing calculator. Enter the equation in the search bar and you get your graph. You can even do multiple functions, just separate them by a comma. You can also zoom and pan in the graphs. You can also do other calculator functions using the search bar.
Here's a screen shot of a quick one I did: y = 4x + 5
According to Google, "This feature covers an extensive range of single variable functions including trigonometric, exponential, logarithmic and their compositions, and is available in modern browsers."
Web 2.0 Calc.com - free online scientific calculator. Available on mobile browser. Can be embedded in your own website or blog too.
Math.com's online calculators - basic, scientific, square root, and more.
eCalc online calculator and math help - has an online calculator along with math help. There are also downloadable calculators, an iPhone app, iGoogle gadget, and embeddable widget for your site. There is also an unit conversion calculator.
Scientific Calculator - online scientific calculator that takes up whole web page. Makes it easier to read and use.
Save money and use free calculators instead of buying them!
Desmos - create media rich educational content online