Wetfire tinder. It is not 'natural' like some of the other suggestions but it comes in its own wrapper, is small, will light from match, lighter or firesteel and burns hotter when wet, apparently. I've found the Maya stick option to be a good one too.
I second that! WetFire tinder is astounding stuff - make sure you fluff it up with a knife, and this stuff burns at the bottom of a full lake. Burns consistently and for a long time, enough to get damp kindling going.
Cheers,
Mungo