Commonly called Hornwort, this is an old standby that will grow where other plants won't, though I've heard it does not grow well in softer water.
This is a fast growing plant. In lower light it is somewhat thin and bright green. As light intensity increases the plant gets bushier and in high light turns red.
Hornwort is a floating plant. Don't try to root it. Pieces placed in the ground will die, the rest of the plant will float back to the surface.
This is a great plant for breeding projects. It works well for hiding the fry from parents or other fish in the tank. It's also very handy for using in bare tanks since it doesn't need gravel.
For more information about Hornwort please see my February 2006 column for TFH's "The Planted Tank," called The Stemmed Plants