LinkedIn Groups are the "happy hunting ground" for recruiters. Many recruiters join groups for the specific professions and industries that are relevant for their clients or employers. They also join the groups that cover job hunting in general.
For job seekers, LinkedIn Groups are a two-fer: you get information and you become more visible to recruiters.
The good news is that you can join up to 50 groups, and you can very easily leave a group that isn't working for you (or after you've found a job). This way, you can test different groups to see which ones work best for you. This list will get you started, but don't stop with these groups. Join LinkedIn Groups for job searching and also for your profession, industry, college, grad school, current/former employer, location, hobby and any other interests.