We used the Markermanager from this location for years https://google-maps-utility-library-v3.googlecode.com/svn/tags/markermanager/1.0/src/markermanager.js. Today it vanished silently and we found here that it was moved to github.
You will find a new packed version here:https://raw.githubusercontent.com/googlemaps/v3-utility-library/master/markermanager/src/markermanager_packed.js. I hope this thread helps others to find it faster then I was.
Please also see Chris Cook's answer, he provides the CDN url...
As Google moved the source over to GitHub a while back, the new GitHub version can be accessed from RawGit by using the following script urls (standard and packed versions):
https://cdn.rawgit.com/googlemaps/v3-utility-library/master/markermanager/src/markermanager.js
https://cdn.rawgit.com/googlemaps/v3-utility-library/master/markermanager/src/markermanager_packed.js
Whilst the above urls (with the cdn prefixes) have no traffic limits or throttling and the files are served via a super fast global CDN, please bear in mind that RawGit is a free hosting service and offers no uptime or support guarantees.
Accessing files maintained via GitHub is covered in more detail in the following SO answer:
Link and execute external JavaScript file hosted on GitHub
This post also covers that, if you're linking to files on GitHub, in production you should consider targeting a specific release tag to ensure you're getting a specific release version of the script.
For example, you could target the 1.2 release of the markermanager library with the following script urls (standard and packed versions):
https://cdn.rawgit.com/googlemaps/v3-utility-library/markermanager/1.2/markermanager.js
https://cdn.rawgit.com/googlemaps/v3-utility-library/markermanager/1.2/markermanager_packed.js
Alternatively, you could download and include the library directly in your project for production purposes.