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
<address>
<street>Roble Ave</street>
<mtfcc>S1400</mtfcc>
<streetNumber>649</streetNumber>
<lat>37.45127</lat>
<lng>-122.18032</lng>
<distance>0.04</distance>
<postalcode>94025</postalcode>
<placename>Menlo Park</placename>
<adminCode2>081</adminCode2>
<adminName2>San Mateo</adminName2>
<adminCode1>CA</adminCode1>
<adminName1>California</adminName1>
<countryCode>US</countryCode>
</address>
</geonames>