Yep, agreed with the southern hemisphere teal, also many of the treeducks are very easy to keep, they also stay in full color year round, and to me make the most beautiful sounds (whistles, A.K.A. whistling ducks) in the bird world to me. 3 require the Fed. permit, the others dont. They are tropical species though and do require winter protection in extremely cold climates, where most other migratory species dont though, so bare that in mind too.
ring teal, marble teal, chestnut breasted teal, the tree ducks, shelducks, sharpwing teal, cape teal, hottentot teal, and many many more, all stay in full color year round, are not Federally protected, and for the most part are easy to keep. There's a whole world out there in this field of aviculture with many beautiful species. Like has been mentioned, most of the northern hemisphere species to have a late summer eclipse plumage where they will look nearly identical to their hens, but their fall plumage is well worth the wait, however most of these are North American species and will require a Federal Sales and Disposal Permit if you intend to sell or dispose of to any other person in any way. If they are for your own person enjoyment, no permit is required. You will be issued a form 3-186 by a licensed breeder upon the completion of the sale for your records. This is all you need to legally keep them (on a Federal level) some states require permits as well. If you do get these and they do reproduce, you do still need to mark them by 1 of the 4 methods covered by the U.S.Fish and Wildlife Service. Right hind toe removal, wing pinioning, web tattoo, or seamless bands.
Goggle around for some images, check various breeders websites, learn as much as you can, there's loads of info out there on them.
There are also 2 major magazines with classified sections for these birds as well.
The Gamebird Breeder's and Conservationist Gazette, and the American Pheasant and Waterfowl Society (A.P.W.S.)
I WOULD SUBSCRIBE TO THEM TOO IF YOU ARE SERIOUS ABOUT GETTING STARTED IN WATERFOWL. EACH HAS VARIOUS ARTICLES EACH ISSUE PERTAINING TO KEEPING OF VARIOUS SPECIES.
OH one more thing too, if you are underage (under 18) a parent will have to get the Federal Permit for you, you have to be over 18 to have it.
Best of luck too you, you will love these things once you get started!!