A pioneer, one of the first, and still one of the best: www.chinesepod.com
Local (Orange County, CA) teacher web site http://www.1jn.com/chinese/index.html
Another longstanding resources site: http://www.chinese-tools.com/learn/chinese
Direct from the PRC: http://learning.chinese.cn/en/ Also: Must Have Software from WenLin (PC and MAC)
Other newer entries: