Windows/Mac/Linux: For the first few days after a new Linux release, grabbing an install image can be tough going. Not so if you've already got an alpha, beta or RC copy—use zsync to grab the necessary updated bits.
The Ubuntu Tutorials blog runs through the two-step process needed to update an ISO from an early release to a final one using the zsync terminal app. It's not installed in most command line interfaces by default, so you'll have to grab it from your repositories on Linux, or follow guides to installing it on OS X with Darwin Ports or add it as a package to Cygwin on Windows.
The server you're downloading from must also offer zsync comparison files for the image you're downloading—luckily, with the release of 9.10 "Karmic Koala, Ubuntu is now doing so, and many other software providers follow suit for big downloads.
Use zsync to update existing .iso images (alpha, beta or RC -> final) [Ubuntu Tutorials]