I would like the ability to create a set of fields for my blog authors to fill out and use them to customize their blogs. With this feature, I could roll out template updates and never worry about re-integrating changes to sidebars and other areas.
For example, I would make fields for Flickr ID, Facebook ID, and Twitter. In the sidebars, I would show their customized widgets if they have entered that information.
If the system was flexible enough, I could specify a custom field for their banner image, a photo to put in their sidebar, a link to their email accounts, etc.
Reported on Movable Type 4.2

Have you looked at the CustomFields feature of MT Pro?
Mike, I use Custom Fields on some other sites, but I don't believe you can set up blog preference level fields. Everything needs to apply to an entry, category, asset, or folder. If I'm wrong, I would love to be corrected.
I see what you mean. I am getting around this on a MT Pro site I am developing is to use local Template Modules. It is not the prettiest solution, but it fits the bill.
I too wish their was "blog level" Custom Fields, though I am not sure that is the best name for it.
Ken
That would make a great feature request.
There's a case extant for allowing custom fields on basically everything.
If anyone wants to tack on a vote, though, it would help.
Quite an extensive list of uses from Jay. I added my two cents worth.