Ok, this isn't open source is the true sense. But, the source code is apparently freely available, so I'll categorize as FOSS.

Here's how its developer describes it: This addin adds the capability of sending yourself email with a certain keyword in the subject and having Outlook automatically create a new page in OneNote's Unfiled Notes section when that mail item arrives. Outlook then deletes the email. It also allows power users to set the filter used for incoming mail items to trigger the code. It provides no UI for the user: "it just works." You can find it linked below.

Outlook to OneNote AddIn

Note that you Outlook 2007 to use this Add-in.