The CAWST website that clearwater cited (thanx for the great resource, btw), also has a page on use of native plants for water purification...

(Also, just a brief safety note on brian's charcoal suggestion - I'm pretty sure everyone here already knows that you make lye by trickling water through hardwood ashes - so unless you want to tan your own hide from the inside out, be sure to rinse the charcoal well before using for water filtration...)