Exodus 16:4
Then said the LORD unto Moses, Behold, I will rain bread from heaven for you; and the people shall go out and gather a certain rate every day, that I may prove them, whether they will walk in my law, or no.
There are several theories on what manna really was. In Catholic School religion class, where we often discussed possible explanations of many miracless, a priest told us of one theory that stated it was the waste excretion of bugs.

I have been able to find some evidence to back up this theory. In California there are bugs called the Plum Aphids. The secretion of these bugs has a sweet honeydew taste and was used by California Indians and purchased by settlers. (Until they learned where the stuff was coming from.) Apparently aphids in general produce a sugary sweet secretion. Another resource states that Trabutina mannipara (Ehrenberg) (Coccidae), a type of aphid, produces so much of this secretion that it is collected by both ants and humans for food.

Columbia Encyclopedia mentions the insect theory, but also mentions theories that manna comes from the gum resin of some trees, or is from a species of lichen.

thecarp proposes quick growing mushrooms that can pop-up overnight and are usually white. I need to see if I can find any references to this.

According to Gritchka, Immanuel Velikovsky believed manna was petroleum rained down from Venus.

ZetaTalk (www.zetatalk.com) puts forth a theory that manna was made of petrochemicals which come from hydrocarbons after being placed under extreme heat and pressure. They say that when the 12th Planet passes it creates ideal conditions for this to occur or during a pole shift. I'm really not sure what they are talking about, they also talk a lot about aliens.

So far I'm going with the bug poop theory.