Google Docs lets you link a spreadsheet to a web form. Entries via the web form (by yourself or anyone else that you have sent the link to) update the spreadsheet automatically.

I use it as a food diary: I enter the food and quantity eaten in the form (via any desktop browser, my mobile or my iPod Touch), and the data is entered automatically into a spreadsheet where formulas look up the calories for the type of food, multiply them by the quantity eaten, group them by date and create a chart and a Google Gadget.

The chart is contained in the spreadsheet and can be published to obtain a snippet of HTML code that you can embed in any web page (see the example below charting the calories I have had during the last seven days).

The chart updates automatically within the spreadsheet. However, if you want the data in the chart that is embedded in your web page to update, you need to republish the one in the spreadsheet. Moreover, if you modify the data range, chart type or settings in the spreadsheet, republishing the chart is not enough, you will also need to replace the code in your web page.

The gadget can also be embedded into your iGoogle homepage, in which case it updates without the need to republish the chart in the spreadsheet.

There are still a few issues that either are not available or I have not figured out, but on the whole this method can already be very powerful and extremely simple to set up.

Yes, I know I said I was going to stop tracking my calorie intake. But I started putting on weight, and this pushed me to find the simpler solution described above, that lets me enter data only once and visualise remaining daily allowance and trends instantly.