With the release of iOS 8, Apple has allowed developers to provide and access extensions. One of the most valuable extensions is provided by 1Password and other password managers (e.g., LastPass) that allow a user to auto-fill password fields with data stored in the password manager without having to leave the original application.
This would be a very valuable option for users who use password managers, and would significantly enhance the security of the application by
1) encouraging users to use unique, complex passwords, and
2) reducing the need for "remember my username and password" to be enabled.
The developer documentation and API from AgileBits (the 1Password developer) is available on github here: