Instacalc: Quick, powerful, shareable calculator
Unlike a full-fledged spreadsheet, Instacalc just has rows, and in each row you can enter a number or bit of math, and the result will be shown at the end of the row as you type. You can refer to values from other rows by their row number (e.g. 5 + R1), or you can give a row a name by putting "rowname =" before it and then refer to it by name (5 + rowname). You can click on the result at the end of a row to hide the row and show only the result, and you can put "//" before a row to make it into a comment, i.e. a text label that doesn't do anything but give information to whoever's using your Instacalc. If you've ever done any programming, all of this is probably sounding pretty familiar, but even if you haven't Instacalc is easy to use. On top of all the various operations, you can also quickly create bar, pie, and line charts, like this country population chart.
But wait, it gets better. Like any good Web 2.0 service, Instacalc lets you embed your calculator in any web page with a snippet of HTML.
Obviously, I'm very impressed by Instacalc and am shuffling it straight into my bookmarks. Head over there and check out some of the sample calculators to quickly get an idea of what it's capable of. Once you've played around a bit, post links to your Instacalcs in the comments.