This is a great site for the 5th-8th grade crowd. Math Moves U has a fun and interactive interface that takes users through worlds of music, sports, fashion and more, all using math concepts. The site is geared at the 6-8 crowd, but advanced 5th grade could work most of the problems on the 6th grade level.
The welcome screen allows the user to create a character and even to customize that character- changing hair color, fur color, and accessories. It has a sign-up/log in feature but logging in is not required.
From there, students can choose to go to several screens, where they can work on flash cards, play math games, take a quiz, take a poll, or work on facts.