A+ Click is a free site full of online mathematics games for students at all grade levels. You can find games on A+ Click by selecting a grade level then selecting a topic. Alternatively, you select just a topic or just a grade level and browse through all of the games. Students do not need to register in order to play the games.
Applications for Education
If you're looking for some good practice activities for your students to use at home, A+ Click could be a good site to link to your classroom blog. Students receive instant feedback on every question. When students answer enough questions correctly they are asked if they want to move up to the harder level or stay at the same level.
H/T to Kathleen Morris and Sue Waters.