Participating in chat rooms and newsgroups is a good way to meet people in your field. Recruiters have been known to hang out in chat rooms on occasion, looking for viable candidates. Peter Weddle warns against silently lurking in these chat rooms in hopes of picking up the names and e-mail addresses of essential contacts. You need to participate in the discussion. Ask questions and offer opinions.
Professional associations host discussions as well.
Spend a couple of minutes every day reading the posts and responding when you have something to contribute. That gets your name in people's minds. Your new contacts may know about jobs before the openings are listed online or in newspapers. It's also vital to network in person. Go to job fairs.