First, let's take down the standard creationist "rebuttal". Then we can get down to business.
A common misconception among the lay public and media is that "vestigial" is equivalent to
useless. Depending on the definition of "useful", this can be true or false, and varies on a case by case basis. However, the biological term, at least as it pertains to evolution, has always meant
"reduced or rudimentary structure when compared with the same structure in another organism".
I think this confusion comes from the way biological "function" is quite specific and differs from the way we use it in common language, e.g. "usefulness". I'll expand on this below.
It could be argued that an inner tube in an aeroplane wing designed by modifying a car tire via evolutionary computer simulation has some "function", i.e. ties it together instead of nuts and bolts; but the fact that it's lacking its "proper purpose", as Darwin put it, in favor of some marginal application
many other structures are more suited to or no application at all, makes it a bona-fide vestige.
Similarly, a broken radio in a car may prevent the dashboard above from collapsing; nevertheless, no one in their right mind would claim that made the radio
any less broken.
For another example, fused wings might make a beetle's back harder and subsequently less prone to smushing; yet they're still
vestigial wings that no longer serve what they're clearly best suited for. Finally, eyes on blind cave fish might be described as "functional" in that they keep their brains from leaking out their eye sockets; but they're still vestigial EYES.
TalkOrigins, as always, subtly rips creationists a few new belly holes in
this well-researched article, including pointing out that Scadding, whom anti-evolutionists love to quote as a supporter of the "vestigial = useless" concept, was quite wrong about the definition of "vestiges", which hasn't been significantly revised in more than a century.
Nevertheless, vestigial organs are now only an accessory line of evidence, not the main ones supporting evolution. They simply don't have the whiz-bang factor of stuff like fossils and ancient retroviral infection fragments shared in genomes of related species.
Vestiges just can't compete here, but they can be an independent test on common descent nevertheless. The standard phylogenetic tree makes a great number of predictions on which ones can be found and which are barred. Mammals, for example, will
never have vestigial feathers in this scenario, as they diverged from the reptiles before birds. Neither will a new species of tropical amphibian be discovered that nurses its young. Sharks will never have vestigial reptilian teeth. And so on, on, and on...
So, where does that leave us? Simply put, there are a great many vestiges in the biosphere. Dandelions, which reproduce without fertilization, nevertheless produce vestigial flowers and
pollen. Humans have vestigial wisdom teeth that don't even sprout in much of the population, much less help in any significant way, and that often create problems by using too much jaw space.
Twenty-eight teeth are fully sufficient for the human jaw, with the extra four causing many a problem in dental history by erupting incorrectly due to our smaller jaws. But just for posterity, how many teeth did
ancient fossil hominids possess?
You may count them. That's 32, with the last ones being similarly oversized as our own wisdom teeth
but nicely-fitting in such a massive jaw. Compare the
Australopithecus africanus and
Homo sapiens jaws in the lower part of the illustration, and take note that the first preceded the other in the fossil record.
Another example; snakes and whales can independently be assigned as descendants of leg-possessing animals based on independent evidence, such as anatomical analysis of their
skulls and
rib cages. In some species of both, a vestigial pelvis can be found. As has been discussed before, whales occasionally resurface with the actual
digit-possessing hindlimbs.
Embryonic comparison confirms this as well, as whales develop temporary hind limb buds.
Finally, early fossil whales had real, bona-fide legs, latter ones used them progressively less, and the last ones had almost undetectible vestiges.
Is the
independent confirmation of
four fundamentally different lines of evidence yet another "coincidence" that common descent would
predict but common descent-denial needs to
explain away?
Possible sign of evolution? 
The British Empire will rise again, spearheaded by Homo Sapiens-Superior 
Linear Mode
