Best Calc is a very useful and easy-to-use extension that allows you to perform complex scientific calculations staying focused of web surfing.
Uses event background page, in this if the extension is not in use - it is unloaded, freeing memory and other system resources.
Detailed help: http://my.opera.com/Serega124/blog/best-calc-help-en
Supports defining your own variables and functions.
Arithmetic: + - * / % **(raising to a power)
Bitwise: << >> >>> & | ^ ~
Logical: && || !
Comparison: > < >= <= == !=
Assignment: = += -= /= %= *= <<= >>= >>>= &= |= ^=