You do not have to go far to find screen savers that educate. Windows users still trying to get the hang of Microsoft Office 2010 can grab a free screen saver that shows tips for using the software from Microsoft's site at bit.ly/QeKT2U. A trial version of the Universal Tutor screen saver from Cool Educational Software is available at www.cooleducationalsoftware.com; an annual subscription is $30.
Recent versions of Mac OS X have included screen savers that display the Word of the Day from the New Oxford American Dictionary, or an RSS Visualizer that pulls down headlines from online news sources. To see what is available in your version of Mac OS X, click the System Preferences icon in the Mac's Dock, click on the Desktop & Screen Saver icon and click the Screen Saver tab in the box.
Plenty of shareware sites offer free screen savers, but be careful when installing any third-party program, as malicious software may be along for the ride. Screen savers can also sap computer resources. If you would rather not lose processing power, go to your computer's settings and set your monitor to turn off after a short period of inactivity.
But if your computer has power to spare, donating its idle time to a group-computing effort like SETI(AT)Home (setiathome.ssl.berkeley.edu) or another from the Boinc project (boinc.berkeley.edu) usually gets you an interesting screen saver while you help scientific causes.