Google has a neat calculator built into its search box. There is a good summary here:

Copy and paste the following queries to see it work:

Arithmetic operations:

sqrt((3*6/9)^2+7!)

Transcendental:

3*sin( 54 degrees )
2*log ( e / pi )

Complex numbers:

(3+7i)*(6-2i)

Unit conversions:

12 miles in kilometers
2400 joules in btu

And now, the coolest part: physics calculations with universal constants and automatic units manipulation:

1 cm^2 * 15 feet per second in gallons per hour

1 kg * speed of light ^2

1/sqrt( permeability of free space * permittivity of free space )

sqrt( 2e-7 N/m * 2*pi * 1 m / permeability of free space )

( ( gravitational constant * earth mass / ( 2*pi / day )^2 )^(1/3) - radius of earth ) in miles

I know there are other tools out there that do this sort of thing (such as my TI-89), but having it on Google makes fun and easy to use for quick calculations.
 PhysOrg.com science news on PhysOrg.com >> Galaxies fed by funnels of fuel>> The better to see you with: Scientists build record-setting metamaterial flat lens>> Google eyes emerging markets networks
 Admin Blog Entries: 5 Yeah that is pretty slick eh? You can also type in common equations and google will display a link to more information on it.

Recognitions:
Gold Member
Staff Emeritus

I have been using the Google calculator here on PF for a long time now. If someone asks a question like "how is the speed of light related to $\epsilon_0$ and $\mu_0$" I will reply with something like this:
 They are related by $$\frac{1}{\sqrt{\epsilon_0 \mu_0}}$$ http://www.google.com/search?hl=en&i...=Google+Search