So far we've done email, SMS and hard copy print incorporated into campaigns. We regularly use the GoToWebinar cloud connector and Dialogue's SMS connection.
It's really all about data connectivity. You can do virtually anything if you can get the data moving.
Some thing's we're looking into now are...
Outbound voicecalls/voicemail
Transactional email (we have several ancillary systems that process internal job traffic. We're working to create data imports from those systems to Eloqua so we can send stakeholders status notifications.)