This post is related to an earlier post on this topic, but it is written (and meant to be read) as a standalone post.

Starting with Mathematica 10.0.1 (and also available in the Wolfram Programming Cloud) you can use "GoogleDocs" as an EmbedCode environment. For example you can do the following:

EmbedCode[ APIFunction[ {"x"->"Number"}, #x^2& ], "GoogleDocs"]

Now open Google Docs (http://docs.google.com) and open a new spreadsheet (select "Sheets" from the top left menu):

Next, click the circled (+) in the bottom right of the window to start a new spread sheet:

Now open the 'Script editor...' by selecting it from the Tools menu:

Switch back to Mathematica and click the 'Copy to Clipboard' button from the EmbedCode object output cell.

Then paste the code into the script editor windows (Code.gs file), overwriting the small code template that is already present in that windows. Then save the file:

Now switch back to the spreadsheet and click on cell A1 and type:

=wolframCloudCall(42)

And then hit enter. The cell A1 will display loading for a short while and then display the answer of 42^2: