RESTful API supporting OAuth2 and JSONP.
App registration is mandatory, non-commercial usage is for free.

Links to RATP (French parisian railways company) OpenData API

This is the site for open data about crime and policing in England, Wales and Northern Ireland.

You can download street-level crime, outcome, and stop and search data in clear and simple CSV format and explore the API containing detailed crime data and information about individual police forces and neighbourhood teams.

You can also download data on police activity, and a range of data collected under the police annual data requirement (ADR) including arrests and 101 call handling.

All the data on this site is made available under the Open Government Licence.

Adds Soundcloud and Mixcloud to music cat

This is a publicly available API comprising of several sub-APIs: company, fundings, geodata, news etc. 
The API itself is proprietary as the company behind charges for its usage but there's a certain amounts of "free calls" for every registered consumer.
Registering the API key is very easy and for free. I have also written a PureScript library to access the API (the backend logic is written in JS, so it can be easily used without PureScript as well).

Not sure if such APIs are welcome here, so feel free to reject this PR.

This was stupid: I have deleted a whole line. Sorry.