I think it's alternanthera reineckii
I got a bit of info on wikipedia a while back. It's an awesome plant! It was growing out of the top of my 10 gallon too. I propogated and put it in the 15 gallon, and it's only got about 4 inches before it gets to the top of that one.
It's not the slow grower I thought it was 
__________________ 29 gallon
Under Construction 20 gallon Flora: Anubias Nana, Microsword Fauna: None yet, starting to cycle 15 gallon Flora: Microsword, Java Moss Fauna: 10 Neon Tetra, 1 Dwarf Gourami (f), 1 Betta (f), 1 Mystery Snail 10 gallon Flora: Java Moss, HC, Dwarf Sag Fauna: 1 Guppy (f), 2 Peppered Cory, 2 Pygmy Cory, 2 Amano Shrimp, Ghost Shrimp, Red Cherry Shrimp 5 gallon Flora: Java Moss, Dwarf Hairgrass Fauna: None |