Just looked through Zendesk's network calls -- looks like it's probably Demandbase. http://www.demandbase.com/landing-page/demandbase-real-time-...
Surprisingly, AdBlockPlus doesn't seem to block it.
Edit: actually it's LeadLander.com as pointed out by NiekvdMaas here http://news.ycombinator.com/item?id=4891764