Duckweed is a common floating plant that really will grow like a weed.The photo at left shows it growing wild in a very shady spot. It can be handy to have around though. Many fish like to eat duck weed. Some Cichlids, large rainbows and goldfish to name a few. It also is a good cover for a tank when it needs some shading. Many anabantoids will use it in their nests.
Other similar plants include:
Limnobium sp.(Frogbit) - Frogbit looks like a large version of Duckweed. It is often sold for ponds but can be used in an aquarium too. Frogbit does need strong lighting.
Spirodela sp. (Great Duckweed) - These are quite a bit larger than the regular duckweeds. They also get a reddish purple underleaf and edges of the top of the leaf in bright light, in dim light they stay smaller and don't get the red. The root structure is different than the regular duckweed also. They are often curly and entangle on things, while the great duckweed has straight short roots.
Wolffia sp. - Wolfia are very small duckweeds. I have never had any luck keeping them as they seem to get lost in the larger duck weeds and eaten by the fish.