Npm. It's meant for Node-js code, but you can use it to make sure modules are installed in your source directory for a website. Aside from npm, there are public CDNs available ( http://www.jsdelivr.com/ and https://cdnjs.com/ ).

For Java (btw), there isn't a central repository, but Maven lets you download the Internet ( http://blog.sonatype.com/2011/04/how-not-to-download-the-int... ) which solves the same problem.