Water Crowfoot (Ranunculus Aquatilis) has the added bonus of little white flowers appearing from now until July and I also like Mares Tail (Hippuris Vulgaris) for it's upright stance.
I have tried frogbit but find that it doesn't last so long, eventually turning yellow.
These guys have a good selection to look at (I have never bought from them):
And if you search eBay for "oxygenating pond plants" you will get a good selection to look at.
Or even just do an image search on Google to get lots of piccies.