PR Newswire has been around since 1954 and serves as a great resource for PR professionals to connect, learn and share their content. I have found that PR Newswire.com has a lot of useful information for the PR world, especially their Knowledge Center tab. It outlines various tips on optimizing various topics including content marketing, metrics, planning, brand exposure, promotion, distribution, and communication strategies, to name a few. Many of the e-books, guides and white papers are free to users as long as you set-up an account.
You can also use this site to send press releases, which can be helpful for those looking for an easy way to distribute, amplify, track and manage their content.