You'll hear a lot of "absolutely no softwoods (pine and ceder) for rabbits", because of the harmful aromatic chemicals in the wood. This is a generalization, because there are ways to remove those aromatic chemicals. I use kiln-fired pine shavings, the heat treatment removing them. I believe that forming the wood into pellets also makes them safe. They're not the cheapest option, though, because you're paying extra for the pelleting process. I can get a gigantic bag of the shavings, which lasts me probably 4-6 months, for $7 at a farm feed mill (or a similar size for $24 at a pet store, because the pet industry is shamelessly overpriced).
If you want to be absolutely safe, hardwood litters (typically aspen) are always safe, as are nearly all things based on paper (Yesterday's News). Clay-based cat litter is never safe, clumping cat litter most of all. Carefresh products are considered the best for rabbits, but they're horrifically expensive.
