Cattails are found by bodies of water in Green Swamplands. They can be placed on a grass next to a water block in any biome once harvested. At the moment, cattails are purely aesthetic.
Biome's O' Plenty:
The Cattails form the minecraft mod Biome's O' Plenty can be used to make Brown dye and Wool.
Brown Dye: 1 cattail = 1 Brown Dye
Wool: 9 Cattail's = 1 Wool