LinkedIn is home to 160,000 companies you can run searches on. Look one of them up and you can find all sorts of useful information, from fresh job listings and accessible HR contacts to new hires (people whose brains you can pick) to people in your extended network who work there (people you a mutual contact can introduce you to via LinkedIn).
You can also view profiles of rising stars with positions you covet to track their career trajectories: What job titles did they hold and companies did they work for before assuming their current positions? Maybe you can't get into the hot shot companies they work for now, but you can target the smaller companies they came from. You can also join any relevant professional groups they belong to on LinkedIn and learn by osmosis.
LinkedIn makes it easy for you to collect and display the testimonials of colleagues, customers, managers and direct reports who know and love you -- right in your profile. You simply fill out a box asking your contact to endorse you, and the contact sends back a reply saying how fantastic you are to work with. Voila! -- instant referral.
A couple of tips for referral seekers from someone who's been on the receiving end of more unwarranted requests for referrals than she can count: Don't ask someone who's never worked with you or never seen a sample of your work for a referral. While your mother might be willing to vouch for your skills, most self-respecting professionals won't. Also, help the person doing the referring by telling him or her what type of job you're looking for and what skills you'd like the person to highlight. That way, you'll get the most relevant referral you can.