Here's some of the ways I have done this this week...
Year 7 (angles lesson)
picture on board:
Year 11 (enlargements lesson)
this video by Dan Meyer http://www.101qs.com/69-super-bear
Year 10 (factorising and solving quadratics)
I paused my IWB on the 'Wreck's Factor' game on Manga High http://www.mangahigh.com/en_gb/games/wrecksfactor (the class had previously played this game for h/w after we went through factorising quadratic expressions)
Year 10 (completing the square lesson)
This caused a great amount of hilarity (mainly with the Mathematics department, my Year 10s didn't think I was being serious! Nonetheless, it worked!)
Year 8s and Year 10:
I gave them one of the www.mathsbox.org.uk 'settlers' target boards to work out
I gave them Dave Gale's QR Code Puzzle task (see http://reflectivemathsteacher.posterous.com/qr-code-puzzle-worksheet)
I found that by doing this I immediately had the attention of my classes as they entered the room and got sat down in their usual place. There was a quicker start to my lessons and I was able to still do all the admin like taking the register, checking h/ws etc whilst they were off and running on the tasks they were given or videos/pics they were to write down questions about. I will definitely continue to ensure that I utilise 'Meerkat Time'!