Seeking open source Geocoding tool which can be used commercially?
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the best suitable option is not geonames OR openstreetmap but both, Geonames is good for city data, and Openstreetmap for streets, if you use both you will get a good dataset for geocoding. that's the goal of gisgraphy, use the two datasets to get the best relevance.
Yes - try out MapQuest geocoding and the MapQuest Open Nominatim service. We've (MapBox) used them on a number of projects and they're great and the TOS are quite reasonable.
US (TIGER data) Geocoder
Geocoder::US 2.0 is a software package designed to geocode US street addresses. Although it is primarily intended for use with the US Census Bureau’s free TIGER/Line dataset, it uses an abstract US address data model that can be employed with other sources of US street address range data.
https://github.com/geocommons/geocoder
Licence GNU LESSER GENERAL PUBLIC LICENSE https://github.com/geocommons/geocoder/blob/master/LICENSE.txt
Rest of the World:
The GeoNames geographical database covers all countries and contains over eight million placenames that are available for download free of charge.
http://www.geonames.org/export/free-geocoding.html