Whether you're conducting research for a well established product or service or a startup that hasn't fully penetrated the market yet, you're going to have to talk to past, current and potential customers, so you might as well use that time effectively.
I've conducted user research for technology, healthcare and environmental companies such as Authentic8, Bombfell, Bump, Cisco, Dividely, Foodspotting, GoLocal, Mozilla, Sanofi-Aventis, Splunk, The Freelancers Union, VMware and Wattbot.
I'm a Resident mentor at 500 Startups and WebFWD.