Besides Siri, are there any applications which integrate with OmniFocus.
For example, in my RSS reader or Twitter client, I would love to send items I wish to read later to OmniFocus instead of Instapaper or Readability.
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 firstname.lastname@example.org.
You will get an automated reply containing a link: click the link and an Omnifocus item will be created.
Users of the Omni-provided Sync Service may like the "Mail Drop" function it offers — rather than the closed loop of sending an email, getting one back and then having to round trip by clicking a link, your email is sent directly into your OmniFocus and picked up when you next sync.
The best way to send stuff to omni-focus on iOS is to simply click on the share extension. That works in most twitter clients and RSS readers. There is also excellent integration with some apps like the Unread RSS reader, so that you can send things directly to omni-focus. Most apps have the share extension button, so it will be your best bet.