That is what VersionEye is doing.
And it has its own REST API
For example: https://www.versioneye.com/api/v2/products/c/git%2Fgit/versions?api_key=Log+in+to+get+your+own+api+token
Response Body:
{
"name": "git",
"language": "c",
"prod_key": "git/git",
"version": "2.6.4",
"prod_type": "GitHub",
"versions": [
{
"version": "2.3.0-rc2",
"released_at": "2015-01-27T22:39:53.000+00:00",
"released_string": "2015-01-27 22:39:53 UTC"
},
{
"version": "2.3.0-rc1",
"released_at": "2015-01-21T01:35:41.000+00:00",
"released_string": "2015-01-21 01:35:41 UTC"
},
...
It is actually free for opensource, and not free for private repos.
It is the only one I know which monitors all opensource.
You have other referentials usually dedicated to a language, like for instance godoc.org (for the golang opensource projects), with its own API, but limited to a language.