This snippet is extracted from my Photo model. I use it to ensure that any uploaded image is constrained to a specified size (resized on save). In my case, I don't need to maintain a "thumbnail" and "fullsize" version, so I just store the resized version to save space.
Because BaseCommand catches all CommandError exceptions and turns them into nicely formatted strings before calling sys.exit(1), it can be tricky to properly test failure. Normally, this is great, but it makes testing that a command fails when we expect to a little harder. That's where this snippet comes in. It redirects sys.stderr to an instance StringIO where we can monitor what's output and catches the BaseException raised by sys.exit(1). Form here, it's trivial to test that a management command fails exactly as you'd expect it to.
This is a HTTP response is use in my application when I want to return a 401. It's pretty simple, but effective for my needs.
This snippet is a piece of middleware that can be used to allow an administrator to create a flatpage that will override an existing URL. Why would you want such a thing? Maybe an admin needs to be able to override individual detail pages of an item list at will and doesn't want to be bothered mucking around with urlconf and restarting apache2. Obviously, it's not the ideal situation for permanent overrides (in that case, you'd be better off writing an exception in the urlconf), but it's good for the temporary, one-off, situations that may arise for whatever reasons.