Ludwigia repens
Ludwigia is another stem plant that needs good lighting, but you will not be disappointed with the results. This plant gives very attractive red to purple leaves. It will also grow out of the top of the tank and sends roots down from the branches. It's easy to clip and replant. The base will also regrow.
For more information about Ludwigia repens please see my February 2006 column for TFH's "The Planted Tank," called The Stemmed Plants.