|BE AWARE that while openaddresses.io may be a convenient index of possible sources, openaddresses.io does not validate entries submitted to it in any form and contains everything from legit open data to data obtain by potentially criminal acts. DO NOT rely on any licence or similar information obtained from openaddresses.io YOU MUST validate licence suitability with the original source and FOLLOW the OSM import guidelines.
Further you should be aware that it is neither clear who operates the openaddresses.io project nor is it in any way associated with OpenStreetMap or the OSMF. With other words: if you are sued for using data from openaddresses.io in your project you are on your own.
See OpenAddresses for other address collection projects
OpenAddresses.io is "a global collection of address data sources, open and free to use" which was created by OSMers User:iandees, User:Ingalls and User:Lxbarth, among others. In fact it started out as a spreadsheet of government address datasets maintained by iandees, but now has an aggregated download, an API, and a website:
github repo (and more readme description)
The meta data (with other words the list of sources) is licensed CC0.
Video of a talk about OpenAddresses - A lightning talk by Alex Barth at SOTM EU 2014.