Simple caching system that caches remote URL contents, addressed by cryptographic hash (SHA-1, SHA-256, etc) and allows retrieval over HTTP.

To use it you simply request specify the hashing algorithm (sha1 and sha256 are currently supported) and hash of the data you wish to retrieve. If it is already cached is returned immediately, otherwise the "X-Cache-URL" HTTP header sent with the request is examined for a URL to fetch to cache this contents.

https://hashcache.rkeene.org/ is running this service.