GoBinder is primarily designed for students (although the student-specific content can be easily changed to support business and/or personal data). It's developed by Agilix, the company who has also developed the FranklinCovey Plan Plus and Tablet Planner products.
There's a new version of GoBinder that is about to enter a public beta test that adds new features including Outlook sync and the ability to store native documents (Word, Excel, etc.) that can either be annotated or opened in their native app for editing. This gives you and "e-binder" where you can collect all of your reference and resource materials and keep them with your notes.
While GoBinder does mirror a lot of what OneNote offers in the way of note taking and organization, it has full scheduling, task, and contact management tools as well.
It's also a $50 product as opposed to $200 (before rebate) for OneNote.
I'm going to be testing the beta release and doing an interview with a product manager at Agilix in the next week or two. I would suggest waiting until the public beta is available (if you're comfortable working with pre-release software) as the new version looks much better than the original release as a GTD tool.
I believe Michael Hyatt has already been using the beta so perhaps he can add more.
If you don't use a Tablet PC, GoBinder, like OneNote, will still be a useful tool but the products have both been designed to take advantage of that form factor with excellent ink support.
UPDATE: As I was posting this, I read that the OneNote SP1 update has been finalized. It will still be a while before it's available to the public but it adds a tremendous amount of new functionality inclluding better support for Outlook and other Office applications.