This is my favorite client. SourceTree is free like all the other tools; it comes from the mind of Atlassian, the well-known company behind BitBucket and other popular services like Jira, and Confluence. SourceTree can handle all kind of remotes, offering facilities (such as remembering passwords) to access most popular services like BitBucket and GitHub.

It embeds the GitFlow way of organizing repositories by design, offering a convenient button to initialize a repository with Gitflow branches, and integrating GitFlow commands provided by the author. The most interesting thing I found at first was that you can enable a window where SourceTree shows the equivalent Git command when you use some of the Git commands through the user interface; in this manner, when you are in doubt and you don't remember the right command for the job, you can use SourceTree to accomplish your task and see what commands it uses to get the work done:

SourceTree is available even for macOS.