OpenStreeMap has a reverse lookup tool too:
http://wiki.openstreetmap.org/wiki/Nominatim#Reverse_Geocoding_.2F_Address_lookup
Example from link:
Query:
http://nominatim.openstreetmap.org/reverse?format=xml&lat=52.5487429714954&lon=-1.81602098644987&zoom=18&addressdetails=1
Result:
<reversegeocode timestamp="Fri, 06 Nov 09 16:33:54 +0000" querystring="...">
<result place_id="1620612" osm_type="node" osm_id="452010817">
135, Pilkington Avenue, Wylde Green, City of Birmingham, West Midlands (county), B72, United Kingdom
</result>
<addressparts>
<house>135</house>
<road>Pilkington Avenue</road>
<village>Wylde Green</village>
<town>Sutton Coldfield</town>
<city>City of Birmingham</city>
<county>West Midlands (county)</county>
<postcode>B72</postcode>
<country>United Kingdom</country>
<country_code>gb</country_code>
</addressparts>
</reversegeocode>