(i've been trying to bring tutor.com to casa through site council, the school district, and/or the pub lib, but it's just too pricey for petaluma at this point. families can subscribe, though, and if i were a parent of a teenager, i would!)
cramster.com offers a limited free membership. you have to sign up for an account, but you get access to textbooks, study guides, study groups, solutions, forums, and most importantly, expert answers.
scroll down on the login page and browse subjects to see free resources.
homeworknyc offers a free dial-a-teacher IM app. teachers are online and available to help with homework monday through thursday from 4:30 to 6:30 pm east coast time.