pf » Google Calculator

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.

That's quite a handy feature, but I was dissapointed to see that it cannot solve simple algebra problems such as: x+5=10. Still very clever on google's part.