The MS-PL, under which Enterprise Library & Unity (e.g.) are licensed, are quite friendly for both corporate and commercial developers, as far as I know:
This license allows developers to take the source code for Enterprise Library, modify it slightly (for whatever reason, even if merely to tweak it for an existing enterprise), and still not have to publish the changes externally.
Other options (such as Apache's license, e.g.) are mentioned here:
I'm by no means believable with regard to licenses, but I need to avoid copy-left licenses when possible. The MS-PL would be a safe choice, as far as I can tell.