The way I describe in this how to is using a Google Spreadsheet Form as it intergrates nicely with Google Sites and all the tools are free and fairly easy to setup. It can even give pseudo moderation as you can delete comments if necessary, but it will not give you the ability to vet comments before they are made (however you could do this using two Google Spreadsheets, one feeding off the other but only showing approved comments).
Make a Google Spreadsheet FormIf you're using Google Apps then use its Google Docs to make a Google Spreadsheet Form. If you're not using Apps use Google Docs to make one. Google Docs provides instructions on how to make a form in the Google Docs Help Centre or you can watch my video on the sub-page.
Add a Second Sheet to sort the CommentsThe form will add the data to the spreadsheet it creates with the latest entry in the next available row of the spreadsheet (furthest down the spreadsheet). Most of us will want to display the newest comment first, so make a new sheet and add the headings from the original sheet: and then in the next row use the SORT() function to display the columns in descending date order (in this example I used
to sort the sheet). You will want to move the new sheet to the left of the form sheet so when you embed the spreadsheet in your site it will show the sorted comments first. You need to publish this spreadsheet to the world if it's a public site (see my video on the sub-page).
Embed the Form in the SiteGo to the page where you would like the form to appear and use the Insert button then Google Spreadsheet Form and either select the form from your list of Forms or enter its URL (see my video on the sub-page).
Embed the Spreadsheet in the SiteAt the same page embed the Spreadsheet either above or below the form, depending on how you want it to look (if you want it to be like Blogger embed the spreadsheet above), using Insert button the Google Spreadsheet and either select the spreadsheet from your list of spreadsheets (see my video on the sub-page) or enter its URL (make sure you use the public URL if published).
You're done! Test the form and you should see the comment you made.
Peter Walsh has also written a how-to on using a formula for formatting the displayed comments: to see it in action go to http://sites.google.com/site/pgwalsh/feedback; and Peter has provided instructions on how to use his formula.
I have also produced a video tutorial to accompany these written instructions.