Slackmath is a neat use of QR codes developed by a Geoffrey Slack, a math teacher here in Maine. On Slackmath you will find dozens of PDFs of algebra problems (Geoffrey told me has plans for other subjects in the future). Each PDF contains QR codes next to the mathematics problems. After students complete the math problem they scan the QR code to check their answers and find a video explanation of how to solve the problem. Watch the explanatory video below to see this is in action.