Mr. Freer suggested http://instacalc.com/. You can't get much simpler than Instacalc. It's very easy to embed the calculator or a link with some preliminary formulas and data as a part of a challenge problem posted to a web page, wiki, or blog in any subject area.

To include simple graphs and charts as part of your lessons , you might consider ChartAll.com which can also be used without setting up a login. Logging in allows you to save your graphs. Browsing the charts, I found several classrooms making use of this simple tool to analyze basic data such as Distribution of Blood Types Within a Classroom. In his comment, Dave from ChartAll.com suggest selecting the "Data from a Google spreadsheet" and then typing the key of a "published" spreadsheet. I loved this idea, but would suggest having plenty of time to learn the limitations and the strengths of this tool if you plan to use it with Google Spreadsheets. After several attempts to graph a 7th grade class project comparing the number of jumping jacks students could perform over time for a Math unit on Variables and Patterns, I concluded that our Google Spreadsheet data would have to be changed from rows to columns to produce useful results with this tool. However, ChartAll's linking and embedding features were quite easy to use and provided a useful tool for creating and sharing basic graphs on line.
For more complex data analysis, try charting the data using Swivel.com which was suggested by ITM reader Kathleen. Our class Jumping Jack data collected using Google spreadsheet imported easily into Swivel, and the site provided us with many ways to look at our data, including disaggregations and summaries we had not considered. If you're a real data geek, Swivel will keep you busy for hours with its wealth of features. Again, we discovered that the graph we were looking for required graphing rows, and Swivel seems

Finally, many of you wrote to me as soon as you spotted the announcement that Google Docs and Spreadsheets launched the anticipated CHARTS and GRAPHS feature. I can't wait to check out this new Google Spreadsheet feature, and promise to report in as soon as I put our Jumping Jack data to the test.