Create a site where users can rate and comment on stories or sites posted by your users just like digg.com
Some of the most popular/best looking Pligg sites:
http://www.techtagg.com/
http://dealspy.org/
http://www.bestweekever.tv/ble/
http://www.videosift.com/
http://www.agentb.com/
http://scooptube.com
http://devbump.com/
http://hugg.com/
http://dealigg.com/
http://www.plugim.com/
http://bandbuzzer.com/
http://www.gamelemons.com/squeeze/