Monster, CareerBuilder, Dice, and the hundreds or thousands of other job boards online, and consolidation sites like Indeed and SimplyHired are great resources to find potential openings in your field. Most job seekers, however, don't use them as effectively as they could for their job search.
There's the obvious benefit of finding appropriate jobs that are posted on them, however, some of the not so obvious ways to look at them are rarely pursued. Sometimes the most valuable information is not the jobs that are posted!
Companies only post some of their openings. It generally costs companies money to post positions on job boards, and the largest ones are the most expensive. So they typically only post a portion of the positions they are trying to fill. With that in mind, it's generally wise to assume that where there's smoke, there's fire!
If a company has several positions posted on a job board, yet none of them seem to fit your background well. There's a good chance they may have additional openings too, some of which may be a better fit.
Check the company's own website to check for other jobs. Use LinkedIn to find contacts and call several people to ask who they know within the organization that might be looking for someone with your background. Google the company name along with some of your key skills to find additional clues.
Find target companies. Some of the most effective job search strategies involve targeting companies, not just ads that are posted. Once an ad is posted online, hundreds or thousands of people see it and a great many apply. A job seeker has a great deal of competition for jobs that are openly advertised. Building relationships with companies that interest you, however, enables you to be in the right place at the right time when an appropriate opportunity arises. Being the only candidate, or one of a very few leads to better results.
Searching job boards for the kinds of companies you'd like to work for and creating a target list can be a very effective way of using a job board even when appropriate jobs aren't listed.
Compare job descriptions to confirm what portions are part of a template, and what's important. Most companies use standard templates for most of their job descriptions and only change details in the Requirements section. Comparing multiple job descriptions from the same company across various fields can help you determine what wording is part of their template, and what is unique about a specific job. Focusing on those unique aspects will help you stand out from other candidates that miss those important points.
Find recruiters in your field. Like the employers themselves, recruiters generally only post some of their openings online. Searching through job descriptions in your field or industry, whether they are a fit or not, can help you find Recruiters, and Firms that specialize in your field. Those recruiters will likely have other positions that have not been posted and some of those may fit your background. It's often difficult to find relevant recruiters for your profession. Looking for them through job boards is an excellent resource to connect.
Compare recruiters job descriptions to companies. While a recruiter can be a great ally in obtaining a position, there are times when it may be better to approach the company on your own. The recruiters client is the company, and their role is to find the best person they can for the position. If they don't believe you're it, or for some other reason you are not able to make progress with them, you can sometimes figure out who the company is through the job description the recruiter has posted.
It's very common to use their clients wording from their own job description in the description they post themselves. Cut and paste a unique sentence or paragraph from their job description and paste it into Google. You may find where the company has the same job description on their own website or in some other spot. You can then pursue the position on your own when you weren't able to learn who the company was from the recruiter. (If the recruiter does share the company name with you, however, do NOT approach the company on your own, it will likely burn a bridge with the recruiter, and ultimately with the company when the recruiter tells them what you did!)
Figure out what's hot. Read job postings from a more objective point of view to learn areas that are in great demand. Often, job seekers only read job descriptions quickly to determine if a position fits their skills or not. However, if you look for commonalities, you can learn a great deal. Are there specific tools, processes, or other skills or certifications that come up consistently that seem to exclude you from consideration? Those may be areas to get additional training or development. Keep track to see how often they really appear and consider how likely it would help you in qualifying for some of those roles. Don't miss the forest when staring at the trees.
Viewing job boards from a broader perspective will help you get more information and better results. Be creative, be curious, and become a detective!