I just happened to have my camera with me the morning I saw this foam under the leaves of a short weed (verbena family, maybe?).

Looked a little closer and saw something there:

What could it be (click on image for a larger view)?

I put the image up on Flickr and almost immediately a photographer there using the name aw c’mon suggested a possible identification: the Túngara frog, a Leptodactylid. He says:
Several types of frogs lay
their eggs in this manner some froth
masses containing just a few eggs
some with much more. I listed one of
the more common species.
The Encyclopedia Britannica says the tungara frog (Physalaemus pustulosus) is
also called Central American mud-puddle frog terrestrial, toadlike frog common in moist, lowland sites from Mexico to northern South America.
The frog is cryptically coloured, its rough brown skin matching the leaf litter in which it lives. Although a mere 25–35 mm (1–1.4 inches) in length, this small amphibian consumes a wide range of insects; unlike many frogs of its size, it does not specifically feed on ants.
Not a positive ID, but really neat!
UPDATE: I’d like to bring Grisham’s comment, made today (September 3, 2008), up here because it calls into question the frog ID, Here’s what he said:
This is a year late, but the froth globules you see are actually “Cuckoo Spit”, produced by young froghoppers, which are known as Spittle Bugs. Look very closely at the “frog”. Notice the compound eyes, the little arthopod leg on the left? See those things that look like webbed feet? Those are its wingbuds sprouting from its thorax. Now you know why these insects are called froghoppers.
Having been emotionally attached to the idea that this was a frog, I resisted the insect idea (you can see my comments below Grisham’s), but on further looking into the matter, I conclude that he’s probably right. Drat! The frogs seem to lay their eggs in mud puddles. Now, I’ve also seen spittle in mud puddles around here, so I guess if I’m going to find that frog, I’d better look in the right place.
Finding a froghopper, though, isn’t so bad. It’s certainly an interesting looking insect!