Officially unveiled at the Apple Worldwide Developers Conference yesterday, iCloud is the company's upcoming. While iCloud in its entirety will be available later this year, parts of it are being enabled sooner, including managing currently purchased items on multiple machines as well as previous purchases from the iTunes store.
The new release of iTunes comes with Automatic Download, a feature that ensures that all devices you have running iTunes are updated with music, apps, and books that you purchase, so an item purchased on one Mac will be downloaded on your other Macs running iTunes 10.3.
Ultimately users will have to wait for iTunes 10.5 and OS X 10.7 Lion to fully implement all iCloud features, but for now this will be a convenient introduction. iTunes 10.3 is available for download either at Apple's iTunes, through Software Update, or from the Apple Support Downloads pages (see below), and will be about 75MB in size depending on the version.
In addition to automatic downloads, the update includes options to download previously purchased items at no cost (though Apple warns that if previous purchases are no longer in the store then they will not be available).