boothcrawler.com is a tool used to source passive candidates. They maintain a database of business cards gathered at technology tradeshows and it is a pretty good recruiter tool, but we primarily use it to find and recruit channel partners.
We sell resume-import tools and most of our customers (recruiters) use Google to find passive candidates. They perform an x-ray search for profiles from social networking sites.
Google has indexed millions of profiles from social networking sites. Our resume-import tool - ResumeGrabber JobSuite - helps you to
=> search Google for targeted profiles from social networking sites
=> find business email address and phone number for each profile
=> export them to an ATS or Excel
This way, you get a actionable, ready-to-use list of targeted candidate profiles.
I will be happy to organize a free product demo if you are interested. We can show you how to build a candidate pipeline by performing an x-ray search in Google.
Meanwhile, you can also download the free trial version of ResumeGrabber from: http://www.egrabber.com/TU423ae97
Passive candidates? Pick up the phone and start calling in your market, network
Don't forget the "usual" sources: your LinkedIn and Facebook network.
It depends on what you're after! For tech skills it's places like Stackoverflow and Github, designers are on Dribbble, healthcare on Zocdocs, and dozens of other places. OR, you could use one of the services that aggregate all of these like us (SwoopTalent), Entelo and others. You should make sure that you can also search employee networks, and make your outbound marketing clever so that people see your jobs where they are, which for passive isn't a job board or your career site!