Github is an extremely popular tool for managing WordPress plugins, and one of the greatest things about it is how easy it makes contributing back to plugins. Even someone who has never touched version control or even thought about it can easily submit improvements or bug fixes to plugins that are hosted on Github. This video gives a quick walk through of exactly how to do that.
To learn more about Pull Requests, read the Github tutorial on them.
Github is one of the best tools you can utilize to encourage community collaboration, and it is also the tool that makes it exceptionally easy for new users to give back to the plugins they love.
Note: this tutorial shows how to use the inline code editor available on Github. I do not recommend using this for major edits. If you plan to make large contributions to plugins, it is highly recommended that you take some time to learn how to use Git so that you can make your edits on your local machine and then push them up to your forked copy of the repository.