Is there a Mac version of MS OneNote? Unfortunately, no, but there are alternatives.
Check out this excellent post at squidoo.com about OneNote alternatives for your Mac.
The best alternative appears to be Growly Notes.
The above is a screenshot, but I am not sure how helpful that is. The link above will take you directly to the product website.
Also check out freeware sites and reviews on Growly Notes:
LifeHacker - http://lifehacker.com/5794236/growly-notes-brings-onenote+like-note+taking-to-mac
Addictive Tips - http://www.addictivetips.com/mac-os/growley-notes-brings-onenote-2010-type-notebooks-to-mac/
Update (January 27, 2013): I am not positive, but I think you can use the OneNote web application through MS Skydrive: http://office.microsoft.com/en-us/web-apps-help/basic-tasks-in-onenote-web-app-HA102843655.aspx?CTT=3
Another option is Pear Note 3 for Mac. Check out the CNET review here: http://download.cnet.com/Pear-Note/3000-2075_4-205406.html
You can also purchase and download Pear Note 3 from Amazon.com: