Prodigy is an awesome site that I use for homework sometimes, but mostly for classroom practice. It is an awesome online game that incorporates math from a 2nd grade level and is set to questions that we are learning in class.
The information for students log in is at the top. Select the homework tab, then click on Math Log in information. There it will tell your students username and password to get into it. You may find the link in many places. To the left hand side of the screen where all of the links are, on the Math Log in info. page, or at questac.org/studentlinks.