When I'm in front of a computer and I need to quickly calculate an arithmetic expression to figure out sales tax, find averages, multiply numbers too large for my brain to deal with, etc, writing a script or a total application would be overkill. Here are the cross-platform solutions that I find myself resorting to:

Python (python.org)

Python's interpreter is convenient. It allows you to enter any mathematical express and it will evaluate it as if it were a line in a Python script. You could even assign values to variables or other data structures, use loops, conditions, and create functions like you would in any ol' script.

Ruby (ruby-lang.org)

Ruby's interpreter has nearly identical features to Python's interpreter, except that the language is Ruby instead of Python...duh

Mozilla Firefox JavaScript Error Console (mozilla.org/firefox/)
In Firefox, selecting "Error Console" from the Tools menu item will launch the JavaScript Error Console window. The Error Console is useful for debugging JavaScript in Web applications. However, much like Python's and Ruby's interpreter, it too can quickly evaluate arithmetic expressions. Its main shortcoming, when compared to the aforementioned Python and Ruby interpreters, is that can only evaluate one line at a time. So you cannot store data in variables, since once you click the evaluate button, your variables are destroyed.

Google (google.com)
Google's powerful searching abilities are one thing, but did you know that it can also perform calculations (Ex: (99*66)²), unit conversions (Ex: 120 kilometres in miles) and even unit+currency conversion (Ex: help the in laws figure out Canadian gas prices ).

Instacalc (instacalc.com)
I've just recently come across this website and haven't explored all of the features yet. However, it looks very promising. Using AJAX to display answers as you type in your expression is a nice touch.

For Windows users, there's also the Power Calculator that's part of the suite of Windows XP PowerToys. I keep it bound to the Calculator key on my keyboard.

better calculators + me = happy