Aspen works well.
The deal with cedar (and all wood shavings) is that there is a natural oil in the shavings. Said oil (which is extra strong in cedar) causes severe respitory problems in small animals and reptiles. This can lead to death.
I always recommend aspen or Carefresh (the later is better but much more pricey) I accually mix both together to make the carefresh last a bit longer for my rabbit and six ferrets.
Ok, that's about it.
thanks
Rusty