Robinia is one tree that has compound leaves like a mimosa and gets seed pods. Some people call it Acacia. The colors range from deep purple to white and all in between.

Here is a link that might be useful: Pictures

I can still picture the roadsides full of the plant you describe when visiting St Simons. I believe you are talking about Purple Sesban(Sesbania punicea).

Thanks.Its sesbania punicea(rattlebox)that I'm seeing.

darn, another non-native tree seeding itself around - apparently especially a pest in wet areas.