What can you redownload from Apple's iTunes store?

You only get one download of songs, videos (includes movies), iPod games and albums ...

iTunes Store: Purchased songs, videos, iPod Games, and albums can be downloaded only once: "When you buy a song, video, iPod Game, or album from the iTunes Store, you are entitled to download it only once...."

On the other hand, iOS apps can be redownloaded as long as they are still sold [1] ...

How to redownload purchased apps from the App Store

Click the Buy button. ... You will then see a dialog similar to one below. Click OK to continue with the download.

The rules variation is a bit confusing. There have been reports, by the way, of Apple allowing redownloading of lost music under some circumstances. Those reports, however, reference a form that no longer exists.  I suspect Apple has discontinued that service.

At 1-2 GB each movies can add up fast. If Apple were to allow redownloads we could delete movies we don't often watch, and retrieve them from Apple. Of course this may not be Apple's decision to make, I suspect the rights holders are the issue.

This policy is one reason I prefer DVDs to iTunes purchases, though I fear the days of the DVD are numbered.

[1] It's not easy to find a record of all purchased apps, but if you keep the invoice emails Apple sends Spotlight will help.