If my feed results are anything to go by, one of my most accessed posts is one I wrote way back when I first started on free Screencasting for Macs. A lot has changed since then (sadly,ScreenToaster came and went, for example), so it's time for an update. These options will work for both Mac and Windows.
Jing: This, of course, is the go-to app for most people. Made by the venerable folks at TechSmith, (makers of Camtasia), Jing is a download that allows you to capture either screen shots or up to 5 minutes of video/audio, making it great for recording quick tutorials. You can host the video on the ScreenShare site, or download it to your laptop. It's quick, easy and fairly intuitive. What it's not is editable (though you can load it into Camtasia to edit. But if you have Camtasia, why are you using Jing?). It also saves the files as flv. (Flash), so you can't load them into iMovie to edit them, either. You can add arrows (call-outs) to screen shots, but not to the video.
Screencast-o-Matic: Not an especially pretty site, but it gets the job done. SOM is an online tool (so no downloads) that captures up to 15 minutes of both audio/video, though you can turn the audio off if you want. Very easy to use. Especially cool, you can opt to place a bright yellow "halo" around the cursor, making it easy to follow. SOM lets you download the final video as either a flash file, MP4 or AVI, which means you can the load the MP4 file into iMovie for further editing. You either host the video on the SOM site or load it onto YouTube.
Screenjelly: Records 3 minutes of audio/video that you can then share via Twitter, Facebook or email, or you can host it on the site.
Screenr: This one is unusual in that it's geared toward Twitter. Again, it records up to 5 minutes of video/audio, that you then post to your Twitter account. Why, I don't know.... It does create video that can be viewed on an iPhone, though.
For Windows users, there are a couple of other options, but I haven't used any of them, so I'll just list them.
CamStudio: Records audio/video, and saves as an AVI file, which I believe you could edit in MovieMaker (let me know if I'm wrong on that.)
Wink: Captures video and audio, and allows you to add explanatory text boxes, buttons and titles.
CaptureFox: Now this one's really different. It's a Firefox add-on, so works through your browser.
Tutorials, obviously. If teachers are going to be absent, they could create a quick video to explain the lesson, which would be a big help for the sub (and eliminate all those "well, the sub didn't really explain") excuses! Students could write and create a weekly (or daily, for elementary) wrap up to post and use for review (or students who missed class).
Here are a couple good how-to's for creating organized, engaging screencasts.
What is Screencasting
How to Make a Screencast
Webinar: Creating Online Tutorials
Need Examples? 100 Awesome YouTube Vids for Librarians (you'll find some tutorials in here).