K-6
Farmville is not a traditional classroom activity, but there are characteristics from many content areas. Agriculture, Math, ELA, Literacy, economics, social studies, etc... Students can become one another's neighbors and help to build a community in which they care for one another's farms. A downfall to this application is that it is blocked by my district and I would suspect it would be blocked by many. If you choose to request permission from you tech department, please note that this is a web 2.0 tool.
K-12
Make phone calls, video chat, can be used for classroom activities and personal use.
Con: Must register and download program, and new district permission.