The OS X Dashboard (and all similar implementations for Windows, Unix, ...) is great for little tasks like looking up a word in a dictionary, or doing simple calculations with a calculator.
Regrettably the calculator widget shipped with OS X 10.4 is really bad. It doesn't honor the regular order of operations, thus entering
1+1*2 leads to
4 instead of
3, and it's very limited in functionality.
I just picked up PEMDAS as a powerful replacement. First of all it honors the order of operations and calculates the way you expect it to. Additionally it offers a wide variety of commonly used functions like square root, sin, log. You can also define variables and use them afterwards.
It also supports different numeral systems like binary, octal, decimal. Unfortunately you can't mix non decimal numeral systems with scientific notation.
Anyway: It's great and you should give it a try.
Update: I just read that REMDAS received an Apple Design Award this year.