Thread Compressor is an add-in to Outlook which removes unnecessary emails within a threaded (ongoing) conversation. Rather than keep each reply, the Thread Compressor works on the assumption that most people reply to mail and leave the original intact hence you can keep the last mail in each branch of a thread, and remove all the others. It removes a remarkable amount of storage space. Thread Compressor, as it is configured by default, WILL DELETE DATA FROM YOUR INBOX. If you choose to download it and use it from here, you do it with the author's blessing, but it's completely at your own risk and Microsoft cannot be held responsible for what it does.

More info: Thread Compressor