http://www.packetattack.com/  If you are currently studying for Cisco certification and need access to your own private lab, tese guys rent it out to you!  Also lots of data and tutorials to help you on your way.
 
http://www.groupstudy.com/  Great resource for Cisco information.  Mailing lists, forum, and newfeed, almost instant replies when you have a problem.
 
http://www.learntosubnet.com/ Just what the title says...Learn to subnet.  A must for anyone in the Networking industry.
 
http://www.r1r2.com/  Free Cisco Lab Access!  Good for CCNA candidates.  Frame lab, etc.  Can be bothersome at peak times as it is a first come first serve basis.  The do have multiple labs though.
 
http://www.routergod.com/  Having trouble understanding Access Lists, OSPF, etc.  Visit this site, it's explained to you in the most unusual fashion.