My ds6's gifted school insists on using xtramath.com too - while I keep insisting that the timed pressure part of it is not fostering his love of math.
At school they use dreambox.com - I'm not thrilled with it - not to mention the closed, proprietary aspects of it.
I've seen better math resources for younger kids at:
http://www.goodsitesforkids.org - tons of stuff there
Try Know Watch Learn as an alternative to Khan. More there for younger ages.
You might also try
www.oercommons.org - for open resources.
