Are there any applications which integrate with OmniFocus?
There's a list at http://forums.omnigroup.com/showthread.php?t=18931. Not a lot, I'm afraid. There's an API available but I guess there's not enough consumer demand for that feature.

For other apps, you might be able to make use of the "Send-To-Omnifocus" feature. From your RSS or Twitter app, forward the item by email to send-to-omnifocus@omnigroup.com.

You will get an automated reply containing a link: click the link and an Omnifocus item will be created.