Floating plants Free or trade up to you.

[fakename]

IT's common name is duckweek, scientific name Lemnaceae. It's an interesting plant! It spreads like wildfire, and can choke off light to other deeper dwelling plants.
It does look pretty cool though, especially in a set up with a lot of surface turbulance where it won't do so well (although it's easy to scoop out regardless...)
Here's a link with some related info....
http://www.mobot.org/jwcross/duckweed/duckweed.htm

Cheers,
Aaron