I've used these for a while and just thought I'd pass on the idea. You can "subscribe" for free and all that happens is that on almost every Sunday, you'll get an email with a link you can click for an e-book you can download and print. Sometimes if the topic doesn't interest me, I'll skip it and delete the email.
Often, though, I'll click the link and print the workbook. I skip the cover pages because for whatever reason, the cover pages are rather cutesy and not reflective of the workbook itself. I would say that the workbooks range from about second to fourth grades in terms of the reading level and activities presented and that they're a good supplement to whatever you might be studying in science and such.
This week, I printed a unit on "simple machines." We might do this packet over the summer or I might save the packet for when we are doing a more detailed study on levers and so on.
When you click the link, you can also look around and play some of their free "funtime games" (see the tab at the top of the page). My fave is called Shisen, but the children like to play Mouse Trap.