Are there any open data sources that can help correlate a given address to a latitude/longitude coordinate? aka geocoding
You mention you want data sources, so I'm going to focus on where you can get information that helps match street addresses to coordinates, rather than APIs. If you can, go with a web service, but there's a certain masochistic pleasure in building your own geocoder from raw data!
OpenStreetMap is the most comprehensive worldwide source, and has good downloading instructions. The biggest drawback is that OSM has been focused on rendering maps, rather than gathering data, which means less visible information like postal codes and house numbers within a street are often not available. Geonames offers bulk downloads that can make up for a lot of the post code problems.
Once you get beyond those, the data starts to get less structured. I'm fond of SimpleGeo's 21m points of interest, but almost any data set that includes both street addresses and coordinates is useful. Knowing more about what your application is would help me point you in the right direction.
FWIW, Karsten W.'s suggestion uses R, so I'm not sure what your tool of choice is, but here're two other options:
1.) A Python client, geopy, that hooks into the geocoders from:
• GoogleV3 • Yahoo! • geocoder.us (free for non-commercial use, accounts available for commercial uses) • GeoNames • MediaWiki • Semantic MediaWiki
I've used it for years. It's solid.
2.) One in Ruby (but I've no personal experience with it) that can use the geocoding APIs from:
• Google • Yahoo! • Bing • Nominatim • Yandex.ru • Geocoder.ca • Mapquest • Ovi/Nokia • FreeGeoIP • MaxMind • ESRI
Firstly, "geocoding" is the name of the task you're trying to achieve (might help with future Googling). The Geocommons geocoder is one example of an open-data-driven geocoder; I think they mostly use US Census data.
(As that example illustrates, a lot of this kind of data is country-specific, so a bit more detail in your question may produce better results.)
Texas A&M Geoservices has a few Geocoding Services/Tools available, however they are for the U.S. only.
The CartoDB service offers geocoding for tables stored in their service. While CartoDB has a subscription fee, the software is open source. In the US, they use TIGER in a PostGIS and for the rest of the world, they use GeoNames.
List of Open/Open Source/Free-ish Geocoding Services and Tools (2017-05-10)
A coarse forward & reverse geocoder in scala.
A widget for finding addresses and landmarks, and flying the camera to them. Geocoding is performed using the Bing Maps Locations API.
Complete Ruby geocoding solution.
Yandex Maps Geocoding Service
The Yandex.Maps service provides a geocoding service for its users. This service can get the coordinates and other information about an object using its name or address, as well as the opposite, using the coordinates of an object to get its address (reverse geocoding).
Gisgraphoid is a library to do geocoding on your Android mobile phone or tablet with the same Geocoder API as Google but with Gisgraphy. Runs on all Android versions, free, no Google API needed by the device, no API key, no limit. You can display the result on OpenStreetMap or Google Maps (Google API key required).
The almost missing PHP library!
From plain text to geometries.
geocodio makes converting US and Canadian street addresses into map coordinates and vice versa as easy as uploading a spreadsheet. The first 2,500 addresses per day are free, and aditional lookups start at a $0.50 per 1,000 addresses ($0.0005/lookup).
Census geocoder provides interactive & programmatic (REST) access to users interested in matching addresses to geographic locations and entities containing those addresses.
FFIEC Geocoding/Mapping System
The FFIEC Geocoding/Mapping System (System) helps financial institutions meet their legal requirement to report information on mortgage, business, and farm loan applications. Geocoding refers to the Metropolitan Statistical Area/Metropolitan Division (MSA/MD), State, County, Census Tract combination (address information) that must be provided for each reported loan application and the System allows institutions to enter a street address to determine the corresponding geocode. The System also provides Census demographic information about a particular census tract, including income, population, and housing data. Please select the appropriate activity year for the address being geocoded. The tract definitions for 2015 and 2016 data are based on the 2010 Census, for 2017 data is based on the 2015 Census. It is critical that the correct activity year is selected when using the FFIEC Geocoding System.
Another upload a CSV -> Spit back geocoded dataset as a service; it appears to offer some services openly, though the details where not very obvious to me.
BING Spatial Data Services
Geocode and Data Source Limits can be found here for Bing Maps Account used with the Bing Spatial Data Services or the Bing Maps Dev Center at
https://www.bingmapsportal.com/ to geocode entities and manage data sources.
GPS Visualizer's Quick Geocoder
Just an easy, in-browser tool if you need to check an address, etc.
GPS Visualizer's Easy Batch Geocoder
GPS Visualizer's Address Locator - Convert multiple addresses to GPS coordinates