The offlineimap Emacs extension allows to run OfflineIMAP inside Emacs.

Download
It is available from the Git repository.
There's no tagged release for now.
Manual
The easiest way to use it is to load it from your .emacs:
(require 'offlineimap)
You can then use M-x offlineimap. It will launch offlineimap in a background buffer.
If you want to run offlineimap when you launch Gnus, you can add the following to your .emacs:
(add-hook 'gnus-before-startup-hook 'offlineimap)