SUPERB! Place the numbers on the number line. Loads of levels. Negative numbers and decimals. Play against the clock for a score. My class's favourite warm up, useful throughout school from year 3 to year 6.
This site is all research based. If you were looking for a new program this would be a good place to start. No lessons or activities, just program evals.
This site contains many lessons that are very practacal. Activities regarding questions we may have wondered about might be found here. For example, how to measure the circumference of the earth. The lessons are described in text form, however, so if you are looking for an interactive lesson that is ready to go then this is not the place to look. On the other hand, there are a lot of really good ideas that are worth exploring.