Creationists often make statements about mutation always being bad and necessarily messing things up. (Usually followed by an inappropriate analogy.) Of course, in reality, many "good" examples of mutations have been discovered or even deliberately engineered.
One particularly nice example can be found in an article from Nature earlier this year (Isalan, M. et al. Evolvability and hierarchy in rewired bacterial gene networks. Nature 452, 840–845 (2008)). In this experiment, Isalan and co. simulated a bunch of random mutations by adding copies of genes to E. coli that were attached to different promoters and thus expressed under totally different control mechanisms than normal.
By creationist logic, each of these should be pretty detrimental to the organism. I mean, if the bacterium was perfectly designed, then this kind of approach (akin in their (misleading) analogies to adding an extra set of indicator lights to a car, controlled by the brake pedal (along with the brake),) should stuff things up. Indeed, even most biologists would assume that most of the constructs would be bad news.
Interestingly, however, this turned out not to be the case. The majority of constructs were tolerated by the bacteria in question. Furthermore, some of them actually conferred a selective advantage in certain conditions, demonstrating that not only can such mutations be tolerated, in certain scenarios they may be retained by the organism. i.e. evolution. (See more in Allison Doerr's article at "The Signaling Gateway".)
Although generally not as striking as this, articles demonstrating the potential power of mutation and selection are published every week. While they do not prove evolution in the unscientific way that Creationists seem to crave, they are certainly and indisputably entirely consistent with it. Despite nearly 150 years since the publication of "Darwin's Origin of Species", no evidence that has been unearthed that clearly contradicts evolution, despite the fact that every fossil dig or sequenced genome has that potential. Evolution is falsifiable - don't believe otherwise. Just because no evidence has been found that shows evolution to be false, it doesn't mean that such evidence couldn't exist, it just means that it's unlikely to be false.