Windows Tip of the Day
http://windows2000.about.com/?once=true&

MacOS Tip of the Day
http://www.osxfaq.com/

Linux Tip of the Day
http://linuxtoday.com/sectip.php3

IE/Internet Tips
http://www.wugnet.com/tips/topicdisp.asp?topic=ie

Office Tips
http://www.wugnet.com/tips/topicdisp.asp?topic=office