FileMakerStandards.org maintains a collection of "authorized" custom functions for use within the Best Practices covered on this site. Through collaboration and consensus, these functions are maintained using version control - which means they are simple text files of custom functions and not maintained within a FileMaker database (although this is possible). This allows for many different authors to modify and update their own custom functions, yet contribute them to these standards. Developers can also help other developers by making suggested changes - therefore improving the quality of the overall collection of custom functions.
Unlike sites such as http://www.briandunning.com/filemaker-custom-functions.com and http://fmfunctions.com/ where you never know when an author has updated a custom function to fix a bug or improve it's functionality, the version controlled functions managed by filemakerstandards.org does not contain duplicates, attempts to follow strong naming conventions, requires comprehensive documentation, uses class prefixes and groupings and allows for local control over your own custom functions.
Using Git & Github.com
Git is a Distributed Version Control System. The advantage to this system is that every person involved in making contributions, to a collection of code, can each maintain their own repository and multiple variations (known as branches). You can co-mingle the "authorized" custom functions with your own. You would do this within a branch of your own forked repository.
In order to start managing your own custom functions along with leveraging the "authorized" custom functions, you'll need to fork the master repository into one which you control. You do this by creating an account on github.com and then creating a fork of the master repository below. You may need to invest some time into learning about branches and making your own branch which includes a mix of your own functions plus the "authorized" ones and a "master" branch of your forked repository where you maintain any functions you have contributed which are accepted.
Master FileMakerStandards.org Custom Function Repository
The master repository for FileMakerStandards.org custom functions is hosted at FileMaker Standards Github Repository
Git GUI Tools
By default, everything that happens with Git happens at the command level. This typically means working on the command line within a console. However, there are GUI tools available to help with the process. Here are some of them.
For those who are more inclined to use the Terminal and simply want a repository management tool, then Gitbox on the Mac is a great solution (it's what I use). Ultimately, you will need to become familiar with Git concepts such as pull and push, but this tool will simplify the process of interacting with git repositories. If you want to read up on how to use git then start here at the Git Documentation