There are a lot of websites that are very beneficial to the students. I have listed a few below.
http://www.amblesideprimary.com/ambleweb/mentalmaths/additiontest.html This is a great website to help the students learn their basic addition facts. There are three different skill levels. After students choose a skill level, the problem shows up, they type in an answer, and then click on "Check My Answer," then they choose a skill leven again and begin another problem.