(note: there are commercial tinders available, but only DIY or naturally-found tinders are included on this page)

• birch bark [1] [2] [3]
  Can be found in the forest. Easy to harvest. Burns the strongest of any forest-found material.
  The best trees for this don't grow everywhere. [1] [2]
• petroleum jelly cotton pads [1]
  Burns the hottest and longest of anything on this page.
  When carrying in a backpack, it's better to store the cotton balls separately from petroleum in a tube.
• fatwood [1] [2] [3] [4] [5] [6]
  Can be found in the forest. Harder to harvest than birch bark. It doesn't burn as hot as birch bark or petroleum cotton, but it's still no slouch. It stores in a pack pretty well. Can also be made at home.
• steel wool [1] [2]
• twine, unrolled [1] [2]
  Stores well in your pack, and does double-duty for lashing.
• resin [1] [2] [2] [3] [4] [5] [6] [7]
  Easy to harvest, but the hardened stuff isn't always easy to find.
• other natural fire-starters
• other natural fire-starters

You can also create a tinder bundle from a variety of forest-found materials. This allows you to use multiple materials that won't work individually, but will work when combined.