For general homework help check out these sites:
Homework Central - This is an excellent site to turn to with many homework questions. The site is divided into collections for grades 1-6, middle-high school, and college. Collections are organized by topic, and are searchable. If you don't find what you are looking for, you can "Ask an Expert" and e-mail your question. This site also has study tips, and practice problem sections. It also has an excellent collection fof reference tools.
Kid Info - The Web's Best Student Homework and Teacher/Parent Resource! - Here's another great starting site! There is an index for students, young children, and teachers and parents. There are many links to subject-specific sites, kid's search engines, and "fun stuff" too.
4th and 5th Grade Student Research Resources - You will find some good links to reference tools and homework related subjects at this site. There are easily understood answers to common questions, and resources for younger students, as well.
The Internet Public Library - When you can't get to the library, here's a library that comes to you! This site has youth and teen collections that are arranged by subject, covering a wide range of topics. There are links to many reference tools as well. This is an especially good starting place if your child needs to do research for a report or homework question.