Andy, Data.com for us is a pay-per-use and DiscoverOrg is a buffet. Plus, DiscoOrg has other enrichment embedded.
DiscoverOrg is an excellent data repository for enterprise. While we're not using the Marketo integration, we've been able to find many qualified opportunities using its various tools. I'd recommend it or RainKing.
Dan, that's a nice stack! We use a lot of the same tools, with AdRoll being the big addition - retargeting is a good driver for us. Perfect Audience (Marin Software) is a good alternative.
We use BrightTalk for webinars, Allocadia for campaign spend management, Reachforce for data append and management and have quite a few integrations to internal reporting systems.
We are using Vidyard for video tracking and engagement, WebEx for webinars (would love to find a replacement), StrikeIron for email validation, and have looked into a few other systems.
Currently we have Webex (agreed about finding a replacement), SurveyMonkey, and soon via Custom Activities API our own product usage data. We use Data.com but it's from SFDC instead of Marketo. Am also hopeful for a Cvent integration soon.
In the same instance we have used InsideView and Influitive.