I am the proud owner of two ferrets. They are litter trained, but the clay found in many litters can be hazardous to their health.
Yesterday's News is a great alternative. It contains no clay and as a bonus is a recycled product- it's made from old newspapers- hence the name. It absorbs odors very well, and a large bag of it is much easier to carry than a bucket of clay based litter.
It is flushable in small quantities, but I find it easier just to toss the whole mess in the garbage. The lack of clay means it doesn't stick to the inside of the litter pan either, so dumping it is a breeze. They do make a ferret specific "Yesterday's News, " but our Petco doesn't carry it and the ferrets haven't seemed to notice.
It won't work for you if you prefer clumping litter, but since we can't use clay, it does the trick for us.