Take multiple desktop screenshots at regular intervals with Auto Screen Capture
Auto Screen Capture is free and powerful, letting you take screen snapshots even on a multi-monitor system (1 to 4 monitors are supported to capture their screens simultaneously).
A nice feature of Auto Screen Capture is that you can define a schedule to let it take screenshots of any or all your monitors at regular intervals automatically. The interface is nothing exciting but contrary to similar apps that are complex, this one is easy to use.
Snapshots can be limited by number and they can be saved in several common formats, such as jpg, bmp, gif, tiff, png, etc. A calendar lets you open photos from certain periods, even making a simple slideshow.
A minor annoyance is that the program saves its captures on its own folder which you are not allowed to change. Of course you can have anywhere a shortcut to that folder, but it would be better if you could change it to a custom favorite folder of yours. There is also the option of easily opening this folder, directly from inside the app.