Search notes:

Nominatim (OpenStreetMap)

Nominatim is an OpenStreetMap API to to geocode addresses, such as Zurlindenstrasse 215. It is clever enough to determine that this address is in Zurich.
The API can be accessed, for example, through the Perl module Geo::Coder::OSM.

Format of query

At least in Switzerland, I had better results when leaving out the zip code.
For example 
Austrasse 24, 8604 Volketswil, Schweiz
found nothing, while 
Austrasse 24, Volketswil, Schweiz
did.

See also

The Nominatim API
The R package nominatim.
libpostal is a C library for parsing/normalizing street addresses around the world using statistical NLP and open data.
The Overpass Turbo IDO extensions for Overpass API: {{geocodeID: …}}, {{geocodeArea: …}} etc.

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