I stumbled across this free app the other day and downloaded it to check it out. Although it is designed to be used by elementary students , it could be used by middle school students who need some help with fractions, decimals and percents. It has blocks from 1/2 to 1/12 with the equivalent percents and decimals located below each fraction tile. The space available is the whole unit so if you want students to see 1/2 + 1/3 they can do it but if you want 3/4 + 1/3, you have a problem. I like that you can use any of the three representations but again it is designed for younger students so it is not an app I would use in the high school.
During warm-ups, I sometimes use a problem called "Which one does not belong?". I give a list of 4 choices such as xy, x^2z, x^2y, xy^2. Students then have to justify their choice. For instance, they might say x^2z because it is the only one with the variable z or xy because neither of the variables is squared. This gives them a way to justify why they made the choice they did. The state tests here often have questions which require students to justify their answers and this gives them a way to practice while developing higher order thinking skills.