Mrs. Tonsfeldt, a third grade teacher here, shared this website with me to check out. We are currently in the process of implementing a new math curriculum and supplements that we may need. She discovered I Know It - a math based website for practice of various skills. This website is COMPLETELY FREE NOW, as it's in beta form and incomplete, but it's SUPER easy for kids to use and SUPER clean and laid out right. CHECK IT OUT!!
Here are a few screenshots from the site to get an idea of the simplicity of it: