There are hundreds of options when you want to post a job, but really in the US there are only few sites that have good traffic: LinkedIn, Craigslist.org, Indeed.com, Monster.com and Careerbuilder.com and the most popular general purpose job sites.
Of course, there are sites that are better for finding specific types of specialties. For example, with designers behance.net and dribbble.com are great options as they are communities where designers hang out. Careers.stackoverflow.com is a good option for developers.
So, here is a spreadsheet which compares the most popular sites to post a job in the USA including its Alexa ranking (which gives you an indication of the popularity of the site) and the cost to post.
You can review the spreadsheet here:
Let us know if we missed anything or if there are more sites you would like to add!