KidInfo

http://www.kidinfo.com/

Even young kids can use this site

Ask an expert

http://www.k12science.org/askanexpert.html

Ask an expert in almost any field. Good for all grades

BJ Pinchbeck's Homework Helper

 http://school.discovery.com/homeworkhelp/bjpinchbeck/index.html

Discovery Channel: more than 700 sites of well organized  terrific links for homework and project assistance.

InfopleaseHomework Center

 http://www.infoplease.com/homework/

Search fairly easily for answers to homework questions

Pitsco's Ask an Expert?

http://askanexpert.com/

Send your questions to experts: better for projects/ papers as questions will not be answered immediately.

The Learning Center

http://vrd.askvrd.org

Amazing site funded by the US Department of Education. Must register to use, but this is free and takes 1 minute.