Update: OmniFocus for the Web is now available, and is a great solution for working with your OmniFocus data on a Windows PC.
While OmniFocus doesn’t offer any direct integration with the Windows platform, and we don’t have any plans to create a client for Windows, many of our Windows customers use our Omni Sync Server’s Mail Drop service to loop their PC into their OmniFocus workflow. By using Mail Drop, you can capture items into the OmniFocus Inbox from any device that supports email.
In addition, if you’re utilizing Outlook for Windows and it syncs with Microsoft Exchange, by configuring Exchange on your iOS device you can set up a one-way migration of Outlook Reminders into OmniFocus through our Reminders Capture feature. While this was originally designed around using Siri to dictate tasks into OmniFocus via the Apple Reminders app, this workflow offers the side benefit of allowing any program that can sync to Reminders.app a way to get tasks into OmniFocus for iOS.
iTunes on Windows also backs up the data on your iPhone or iPad when you plug it in, so while this doesn’t give you a way to directly interact with your database from the PC, it’s a great measure to ensure your data is backed up in case something untoward happens. As a measure of redundancy, it might also be worth synchronizing with Omni Sync Server or another WebDAV server, so that you’ll have a live copy of your database living somewhere online in the event that your mobile device is damaged or stolen.