Yes.

Last example: Angry Birds

It wasn't in the Market at first. And many seem to do it since getjar.com was down some hours after Rovio announced via twitter that you could get it there.

I host pre-beta versions of my Apps on my own servers and give friends (and their friends) access to it. This would be impossible with iOS. I would have to know and register the hardware IDs of all devices in advance.

I have indeed installed apps this way. I admit that most apps I've installed are from the market, although that's missing the point. Frequency doesn't matter as much as the actual ability to make the choice independent of Google (or indeed any single corporation).

If you think this doesn't matter in practice, consider Apple vs Google Voice on the iPhone. Apple rejected (or "didn't approve") voice, leaving iPhone users with no way to access it. Users would be better off if Google could have hosted a Google Voice package external to the app market.

I've done it for beta apps and some other things I've found on websites. Often there's a QR code to scan to download the file that's not in the market.

Swype keyboard is an example of an app that's not in the market but can be installed via APK. For most users, it's not a big deal, but I think that's mostly because the Market has far fewer rules than the AppStore, so even apps that require root access can be uploaded to the Market.

I'll often use the barcode reader app to read QR codes for apps, some of which take you to the market page, others are links directly to apk files.

Only to keep SL4A updated. It hasn't made its way to the marketplace yet.

http://code.google.com/p/android-scripting/

That's how I installed Fennec.

I've never done it.

Quite a bit.