You can use GeoNames' FindNearestAddress.
It takes a latitude and longitude parameter and returns the nearest address in XML format.
From the example:
http://api.geonames.org/findNearestAddress?lat=37.451&lng=-122.18&username=demo
Roble Ave
S1400
649
37.45127
-122.18032
0.04
94025
Menlo Park
081
San Mateo
CA
California
US