I have previously been a user of CamStudio when it comes to screencasting. It's worked well in the past, but downloading it nowadays makes your anti-virus suite go haywire over the included malware. So it felt like a good time to shop around for an alternative.
There are plenty of alternatives when it comes to screencasting. TechSmith have a few good ones, like Camtasia and Snagit. There's even a few that doesn't require any installation, like Screenr and Screen-o-matic.
But one that really caught my eye was ActivePresenter that offered a free edition which contains pretty much everything I need from a screencast program; easy-to-use recording options, a great tool-set for editing (including pan-and-zoom and highlighting) and export in standard high-quality formats.
If you're going to use the program for some serious teaching you might want to go for the standard or professional editions. But for standard screencasting you come a very long way using the free edition.