Quick and dirty list of WiFi AP's and connect to it. Only WPA/WPA2 tested.

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:installLocation="auto" package="dk.ostebaronen.wifishizniz" android:versionCode="1" android:versionName="1.0">
  <uses-sdk android:targetSdkVersion="17" />
	<application android:label="WiFiShizniz"></application>
	<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
	<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
	<uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />
</manifest>

Main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:minWidth="25px"
    android:minHeight="25px">
    <ListView
        android:minWidth="25px"
        android:minHeight="25px"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:id="@+id/listView1" />
</LinearLayout>

wifi_dialog.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content">
    <EditText
        android:id="@+id/password"
        android:inputType="textPassword"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="4dp"
        android:layout_marginLeft="4dp"
        android:layout_marginRight="4dp"
        android:layout_marginBottom="16dp"
        android:hint="Password" />
</LinearLayout>

WifiActivity.cs

using System;
using System.Linq;
using System.Threading;
using Android.App;
using Android.Content;
using Android.Net.Wifi;
using Android.Runtime;
using Android.Widget;
using Android.OS;

namespace WifiShizniz
{
    [Activity(Label = "WifiShizniz", MainLauncher = true, Icon = "@drawable/icon")]
    public class Activity1 : Activity
    {
        private ListView _listView;
        private ArrayAdapter<WifiEntry> _adapter;
        private string _selectedSsid;

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);

            _listView = FindViewById<ListView>(Resource.Id.listView1);
            RefreshWifiList();

            _listView.ItemClick += ListViewOnItemClick;
        }

        private void ListViewOnItemClick(object sender, AdapterView.ItemClickEventArgs itemClickEventArgs)
        {
            var wifiItem = _adapter.GetItem(itemClickEventArgs.Position);

            _selectedSsid = wifiItem.SSID;

            if (wifiItem.Encryption.Contains("WPA"))
            {
                OnCreateDialog(WpaDialog).Show();
            }
            else if (wifiItem.Encryption.Contains("WEP"))
            {
                OnCreateDialog(WepDialog).Show();
            }
            else
            {
                //Other, captive portal, open network, etc.
            }
        }

        private void RefreshWifiList()
        {
            var wifiManager = GetSystemService(WifiService).JavaCast<WifiManager>();
            wifiManager.StartScan();

            ThreadPool.QueueUserWorkItem(lol =>
                {
                    while (true)
                    {
                        Thread.Sleep(TimeSpan.FromSeconds(3));

                        var wifiList = wifiManager.ScanResults;

                        if (null == _adapter)
                        {
                            _adapter = new ArrayAdapter<WifiEntry>(this, Android.Resource.Layout.SimpleListItem1,
                                                               Android.Resource.Id.Text1);
                            RunOnUiThread(() => _listView.Adapter = _adapter);
                        }

                        if (_adapter.Count > 0)
                        {
                            RunOnUiThread(() => _adapter.Clear());
                        }
                            

                        foreach (var wifi in wifiList)
                        {
                            var wifi1 = wifi;
                            RunOnUiThread(() => _adapter.Add(new WifiEntry { SSID = wifi1.Ssid, Encryption = wifi1.Capabilities}));
                        }

                        RunOnUiThread(() => _adapter.NotifyDataSetChanged());
                    }
                });
        }

        private const int WpaDialog = 0;
        private const int WepDialog = 1;

        protected override Dialog OnCreateDialog(int id)
        {
            var customView = LayoutInflater.Inflate(Resource.Layout.wifi_dialog, null);
            var builder = new AlertDialog.Builder(this);
            builder.SetIcon(Android.Resource.Drawable.IcMenuPreferences);
            builder.SetView(customView);
            builder.SetTitle("Set WiFi password");

            switch(id)
            {
                case WpaDialog:
                {
                    builder.SetPositiveButton("OK", WpaOkClicked);
                    builder.SetNegativeButton("Cancel", CancelClicked);
 
                    return builder.Create();
                }
                case WepDialog:
                {
                    builder.SetPositiveButton("OK", WepOkClicked);
                    builder.SetNegativeButton("Cancel", CancelClicked);

                    return builder.Create();
                }
            }
  
            return base.OnCreateDialog(id);
        }

        private void WepOkClicked(object sender, DialogClickEventArgs e)
        {
            var dialog = (AlertDialog) sender;
            var password = (EditText) dialog.FindViewById(Resource.Id.password);

            var conf = new WifiConfiguration();
            conf.Ssid = "\"" + _selectedSsid + "\"";
            conf.WepKeys[0] = "\"" + password + "\"";
            conf.WepTxKeyIndex = 0;
            conf.AllowedKeyManagement.Set((int)KeyManagementType.None);
            conf.AllowedGroupCiphers.Set((int)GroupCipherType.Wep40);

            var wifiManager = GetSystemService(WifiService).JavaCast<WifiManager>();
            wifiManager.AddNetwork(conf);

            foreach (var network in 
                wifiManager.ConfiguredNetworks.Where(network => network.Ssid.Contains(_selectedSsid)))
            {
                wifiManager.Disconnect();
                wifiManager.EnableNetwork(network.NetworkId, true);
                wifiManager.Reconnect();
            }
        }

        private void WpaOkClicked(object sender, DialogClickEventArgs e)
        {
            var dialog = (AlertDialog)sender;
            var password = (EditText)dialog.FindViewById(Resource.Id.password);

            var conf = new WifiConfiguration();
            conf.Ssid = "\"" + _selectedSsid + "\"";
            conf.PreSharedKey = "\"" + password + "\"";

            var wifiManager = GetSystemService(WifiService).JavaCast<WifiManager>();
            wifiManager.AddNetwork(conf);

            foreach (var network in
                wifiManager.ConfiguredNetworks.Where(network => network.Ssid.Contains(_selectedSsid)))
            {
                wifiManager.Disconnect();
                wifiManager.EnableNetwork(network.NetworkId, true);
                wifiManager.Reconnect();
            }
        }

        private void CancelClicked(object sender, DialogClickEventArgs e)
        {
            
        }
    }

    public class WifiEntry
    {
        public string SSID { get; set; }
        public string Encryption { get; set; }

        public override string ToString()
        {
            return SSID + " : " + Encryption;
        }
    }
}