The answer depends on your goals. Do you want to generate stories online? In broadcast (radio/TV) media? Print media? Industry publications? Or maybe you want to position yourself and/or your company as a leader in your field? Or drive traffic to your website or create an SEO boost for your site? Or meet financial disclosure regulations?

I'm going to assume your goal is to generate news stories. The best way to earn media attention is through very specific targeting of your press release to individual journalists who cover your beat (the subject area your press release is about, such as business, or womens' issues or motorsports, for example) AND the topic of your specific release. I don't know of any press release distribution services can promise that (even the paid ones). Their targeting is not very narrow, and most don't have the ability to target recent related stories.

Most public relations pros like me subscribe to a media database service that allows us to do this granular level of targeting. Unfortunately, if you're interested in free distribution, the fees for these databases are probably out of your range. However, you can identify journalists manually by making Google News your friend and searching for recent stories about your topic. Most online news outlets post journalists' email address or Twitter handle, or offer an online form for contacting them.

Most free online press release posting sites have gone the way of the rotary phone. Google used to look favorably on links in press releases for SEO purposes, but that's no longer the case, so there are few left to choose from. The best way to find one is to search Google for "free SEO press release distribution 2016."

Then, to make sure the site's releases are picked up by Google News, search for the site at https://www.news.google.com. Let's pretend our distribution site is called FreelancePR.com. In the Google News search bar, you would type the following, without the brackets: [site:freelancepr.com] (yes, there's no space after the colon) and look for anything that seems to be a press release. Make sure to narrow your search results down to items posted in the previous week (because free press release sites are often de-indexed.)

If nothing from the site shows up in Google News, it's probably not worth trying. It's not there because Google doesn't consider it to be an authoritative source of content.

I'm not currently aware of any truly free PR distribution sites that are indexed by Google News. As others have said, you will probably have to pay to post your news online.