DoubleTwist, a fantastic little alternative to iTunes, has just become a big alternative. The Mac app is no longer just a stand-in library app: it now features music buying courtesy of Amazon, and lets you sync that music to pretty much any device you own, be it Palm, Blackberry, Sandisk, or iPod/iPhone. (The software does not yet support devices running iPhone OS 3.0+, a curious caveat that should be remedied soon, according to the DoubleTwist Web site.)
The ability to buy music puts DoubleTwist almost fully on par with the iTunes experience (though Amazon does not provide movie or TV rentals and sales, as Apple does.) And since Amazon's album prices are frequently cheaper than iTunes, you can buy more for your money.
Come to think of it, everything about DoubleTwist is more economical, down to the visual design of the app; simple and clean, it makes iTunes look even more bloated than before.
As Gizmodo notes, the DoubleTwist/Amazon alliance isn't mere competition—it's outright war. DoubeTwist's founder has been a voluble Apple critic, and even went so far as to buy ad space on the side of an Apple store in June. DoubleTwist for Windows is reportedly in the works and coming soon.