pewMaster is a free-to-use (fair-use) masterserver that can be accessed via HTTP.
That way it is usable by almost any language/framework. It keeps a list of open sessions (until they get closed or time-out) that can be queried by game/app.
pewMaster does NOT do NAT-punchthrough. It's just for listing sessions and exchanging ip-addresses.
The API is quite simple (CRUD).
The sessionhost has to create a session to get an sessionid. Afterwards the sessionhost has to update the session, otherwise the session will get dropped after 5 minutes. When the session is closed by the sessionhost it can close the session to keep the listing clean.
The client can only list the sessions. (max. 1000 results)