With Outlook, there is an option to retrieve the email if the person hasn't opened it.
With our system it doesn't work perfectly though and the recall sticks another email in the receiver's inbox. if they click on the recall one before the email to be recalled, then it works and they will see the other one deleted automatically. The recall email also deletes itself.
If they click on the email to be retrieved first then when they click on the recall email, it deletes but the other email remains.
Good luck with the email that your *ahem* work collegue sent :p
As Tenaka said, if you have the standard installation of Outlook you can recall messages. Open the original message that was sent in error, and click on tools, there should be an option to recall the message. (sorry its a bit vague but my current installation of Outlook doesn't let me do it)
As Tenaka also said, it is visible as the recall goes into the other user's inbox and a fast moving person will see them and get to read them. 99% of people will never know that they had an e-mail and it was recalled however.