Recs.
Updated
Everyone knows and has at least once used GitHub. It is the largest code host on the planet and has over 21.4 million and many users. It is unarguably the most widely used tool by developers worldwide.
SpecsUpdate
Pros
Pro Anyone can submit contributions as pull requests
Any user can fork a project, make changes, and submit those changes back to the original project as a pull request. Pull requests allow for instant code reviews, where project owners, maintainers, and other interested parties can view and comment on your changes, then merge them back into the original project with a single click.
Pro Feels and acts like a "social network for coders"
GitHub sometimes acts also as a social network for coders. It has different features that allow users to follow or star their favorite projects. The user's account page is like a Facebook wall with projects, pull requests, forks, pushes and merges all displayed. The main dashboard after logging in also displays news from followed developers or projects.
Pro GitHub makes it easy to find open source projects
GitHub is the largest host in the world for open source projects. Developers from all over the world fork and work on countless projects hoset on it. For this, GitHub has some excellent features that help both new and experienced devs to find and commit to open source projects of any kind.
Pro Nice and usable UI
GitHub's UI is clean and intuitive. Each view is designed to not fill the screen with useless information.
For example, the repository view displays only the most crucial data about that repo - on the top it displays the number of commits, branches, releases and contributors. When clicked, each of them will take the user to a page that displays more detailed information.
Cons
Con GitHub is known for deleting files they don't like despite not breaking the sites TOS
GitHub has in the past removed content from their site that is not breaking their TOS. Examples such as the E Plus Equality project as well as the operation disrespectful nod repository have been removed despite not breaking any site rules and appear to have been done out of employee distaste.
Considering the amount of content allowed on the site that is full of rude or vulgar humor, to single out these projects for removal shows employee politics and agendas take the front seat over an open and tolerant environment. Which is quite contradictory to open source ideals.
Con Becoming the "center of the libre/open source world" → risk of monopoly
Although Git is a DVCS, it's easy to get tied to GitHub especially when using one of the many external services that only integrate with it. There are not good monopolies so it's a serious risk for the whole ecosystem to tie itself to this non-{libre/open source} non-selfhostable platform.