A quick and dirty address validator using Google's Geocoding API. Could be easily expanded upon.

AddressPOCO.cs

using System;
using System.Collections.Generic;
using System.Text;

namespace Validation
{
    public class AddressComponent {
        public string long_name { get; set; }
        public string short_name { get; set; }
        public List<string> types { get; set; }
    }

    public class Location {
        public double lat { get; set; }
        public double lng { get; set; }
    }

    public class Northeast {
        public double lat { get; set; }
        public double lng { get; set; }
    }

    public class Southwest {
        public double lat { get; set; }
        public double lng { get; set; }
    }

    public class Viewport {
        public Northeast northeast { get; set; }
        public Southwest southwest { get; set; }
    }

    public class Geometry {
        public Location location { get; set; }
        public string location_type { get; set; }
        public Viewport viewport { get; set; }
    }

    public class Result {
        public List<AddressComponent> address_components { get; set; }
        public string formatted_address { get; set; }
        public Geometry geometry { get; set; }
        public string place_id { get; set; }
        public List<string> types { get; set; }
    }

    public class AddressObject {
        public List<Result> results { get; set; }
        public string status { get; set; }
    }
}

AddressValidator.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using Validation.Properties;

namespace Validation
{
    public class AddressValidator{
        private readonly string _apiKey;
        private readonly string _apiUrl = "https://maps.googleapis.com/maps/api/geocode/json?address={0}&key={1}";

        public AddressValidator() {
            _apiKey = Resources.google_geocoding;
        }

        public string GetFormattedAddress(string incompleteAddress) {
            WebRequest request = WebRequest.Create(String.Format(_apiUrl, incompleteAddress, _apiKey));
            using (WebResponse response = request.GetResponse()) {
                string text = new StreamReader(response.GetResponseStream()).ReadToEnd();
                AddressObject address = JsonConvert.DeserializeObject<AddressObject>(text);
                return address.results[1].formatted_address;
            }
        }

        public bool IsAddressValid(string address) {
            WebRequest request = WebRequest.Create(String.Format(_apiUrl, incompleteAddress, _apiKey));
            using (WebResponse response = request.GetResponse()) {
                string text = new StreamReader(response.GetResponseStream()).ReadToEnd();
                AddressObject validatedAddress = JsonConvert.DeserializeObject<AddressObject>(text);
                return validatedAddress.status == "OK";
            }
        }

        public List<AddressComponent> GetAddressParts(string incompleteAddress) {
            WebRequest request = WebRequest.Create(String.Format(_apiUrl, incompleteAddress, _apiKey));
            using (WebResponse response = request.GetResponse()) {
                string text = new StreamReader(response.GetResponseStream()).ReadToEnd();
                AddressObject address = JsonConvert.DeserializeObject<AddressObject>(text);
                return address.results[1].address_components;
            }
        }
    }
}