However, if you're a regular Twitter user but you know colleagues who aren't, I really recommend sharing this list with them, as it may be that they've never heard of some.
2. A+ click
3. Three-act maths
I'm less willing to just let pupils go completely along their own train of inquiry, as is suggested by the model - I think a bit of careful steering is necessary so that pupils end up meeting the objective you've set - but with this caveat, the Three Act Maths stuff is great for making a topic richer and introducing elements of critical thinking and problem solving.
I've just noticed that the NRich team are also developing another site, Wild Maths, which seems to be more pupil-directed than the original site. I'll be interested to see how it develops.