Good news! Google now offers a cached, compressed copy of Prototype on its high-speed content distribution network via the AJAX Libraries API.
You can either link to the source code directly:
Or you can use Google’s API:
More information is available from Google’s documentation.
When a specific version of Prototype is delivered to your browser, it will be cached for one year and served with the proper compression headers. That means that most users of sites which link to Google’s copy of Prototype will incur a ~30 KB download only once.
Special thanks to Dion Almaer and the team at Google for making this possible.