Google calculator
Now Google has added a calculator. But it isn't just a normal math calculator, it also does conversions. You can use it just like a normal Google query. "1 smoot in feet", (I don't even know what a smoot is...) or "1 cubit in feet". I had a rough idea how long a cubit was. Google says it is 1.5 feet. So, just in case you have a bible that doesn't list Noah's Ark in feet, but still lists it in cubits, you'd now be able to calculate that it was 450 feet long, 75 feet wide and 45 feet high. :-)

But in addition to doing calculations of things you may not use everyday (like cubits and smoots), it also does normal type conversion and mathematical calculations:

To use Google's built-in calculator function, simply enter the expression you'd like evaluated in the search box and hit the Enter key or click the Google Search button. The calculator can evaluate mathematical expressions involving basic arithmetic (5+2*2 or 2^20), more complicated math (sine(30 degrees) or e^(i pi)+1), units of measure and conversions (100 miles in kilometers or 160 pounds * 4000 feet in Calories), and physical constants (1 a.u./c or G*mass of earth/radius of earth^2). You can also experiment with other numbering systems, including hexadecimal and binary.