Do yourself a favor and check out InstaCalc.  It’s a very clever idea on the age old calculator.  Not only does it do computations on the fly, but the page loads freaking fast and you can share your calculations with others. It even support charting, like bar and pie charts.  Best part is that it supports variables, making ‘complex’ calculations easy to follow.

For example, check out the YouTube acquisition analysis or a comparison of the Zune vs the iPod.

Check the feature tour out here to see the power that’s available (you will be surprised at the richness of functionality).

Go go InstaCalc!