Dry grass works well but it has to be 100% dry in order to catch spark and start burning. Itīs good idea to mill it using your fingers in order to prepare as small pieces as possible - almost a powder.
Like others mentioned another great tinder is the birch tree bark. But it has to be extremly thin peel - the type of the bark that peels itself off of the surface of the birch. It catches the spark very well and itīs quite water resistant.
I am wondering if anyone has tried the Maya Dust. Of course, it is not a naturally occurring material either.
I use Maya sticks as my favourite tinder. According to the description it cotains 80 % of resin. I have to confirm that itīs literally impregnated with it. It should be therefore waterproof. I have been testing it quite a lot and itīs great tinder. It burns for a long time. I have yet to test its waterproof attributes. I have made several variations of a tinder from the Maya sticks - from the sawdust to the bigger pieces (in the second bag from the left itīs mixed with birch tree bark):