Plants listed by placement.

[IRONHEAD]

This is a good list of plants sorted by location in tank.
ie. foreground, midground and so on.
Here is the link.

http://www.e-aquarium.com.au/foreground-plants.htm

(update)
This site is great too. Not only does it classify plants by size but it shows good pictures of them too.

http://www.aquahobby.com/e_aquarium_plants.php

I realize this site has a plant database so I just want to let people know that.

[cocoa_pleco]

that is good! thanks for sharing!

[IRONHEAD]

Your welcome. Also a good site to find nice layout designs to get ideas is here:

http://showcase.aquatic-gardeners.org/2007.cgi

They list the plants they used in each aquarium so it is really helpful. And if you happen to have a nice aquarium you can submit it here. But it needs to be nice.