By now you've probably seen Google Instant which displays search results as you type. Google claims that it helps you search faster because you don’t have to finish typing your full search term, or even press “search.” I think that this video of Billy Joel's "We Didn't Start the Fire" on Google Instant pretty much sums its up
iTunes Instant is basically a search engine for iTunes, which the creator said he made because, “Search within iTunes.app is extremely slow and cluster, I’ve never used it. So I use Apple’s Search API instead, and develop[ed] this app in less than three hours that will make your life better.” A quick test of the app reveals that it does work pretty well — a search for relatively obscure jams returned relatively accurate results in near real time. (However, it doesn’t quite work when searching for both artist and song.)
Stephen Ou created iTunes Instant in less than three hours hoping to solve two major problems in iTunes.app search: cluster interface and slow speed. According to Ou it's "not just another instantization app, it is clean and fast, and it really fills in the gap that keeps people away from iTunes native search"