This is a nice blog curating the week's interesting internet finds for mathematics. Might be a cool thing to have on your reader for catching useful classroom activities.
Short (3 to 7 mins) videos about famous numbers and cool graphics eg batman curve, kanji symbols for numbers etc. Very good for enrichment, but the presenters are sometimes a bit geeky.
Great visual interactive scientific scale of items in our universe. Great for powers of 10 and I also saw a useful exercise with trig functions in there!
Interactive practice problems based on American algebra 1, geometry, and algebra 2 syllabi. Instant feedback to students. Good variety of problems on each page.