Summary: Allows you to work out simple arithmetic and trigonometric equations right from the address bar. Syntax: You can type something like:

math 10+20*30/40

and in the drop down suggestions menu will be the answer (notice the "= 25"):

math 10+20*30/40 = 25

You can type trigonometry too like:

math 10 + cos(rad(45)) + log(10) + PI*180 / (2^4 + atan(1))

Caveats: Note that (^) is a "the power of" operator, and not a bitwise-or operator. And that (~) is the "negative" operator, so negative 2 is "~2" and not "-2". Supported constants: Euler's constant (E), Natural log of 2 (LN2), Natural log of 10 (LN10), Base 2 log of E (LOG2E), Base 10 log of E (LOG10E), Pi (PI), Square root of 0.5 (SQRT1_2), Square root of 2 (SQRT2) Supported (one argument) functions: abs(x), acos(x), acosh(x), asin(x), asinh(x), atan(x), atanh(x), cube root (cbrt(x)), cos(x), cosh(x), radians to degrees (deg(x)), E to the power of x (exp(x)), log(x), log10(x), log2(x), degrees to radians (rad(x)), sin(x), sinh(x), square root (sqrt(x)), tan(x), tanh(x). Additional Features:

If you type an equation and press the "enter" key, the result will be saved and an Address Bar icon will appear, which you can click to pull up the last 30 saved equations. The history is cleared when you close the browser.
If you use down arrow key to select the row with the answer, the address bar will be filled with nothing but the answer, allowing you to copy and paste it.

Competition: Wait a minute. How is this better than me just typing Javascript in the address bar or using a bookmarket? Firstly, the math is calculated in real-time and warns you whether the equation up to that point is malformed or has mismatching brackets. With Javascript you have to press enter to find out if there were errors and each time the cursor jumps to the end of the sentence. Secondly you can perform the whole thing with the keyboard, which you can't with bookmarklets. Then there's the history feature which saves the last 30 calculations.