using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;
using System.Drawing;
public partial class _Default : System.Web.UI.Page
{
    private PeopleEditor objEditor;
    protected void Page_Load(object sender, EventArgs e)
    {
        objEditor = new PeopleEditor();
        objEditor.AutoPostBack = true;
        objEditor.PlaceButtonsUnderEntityEditor = true;
        objEditor.ID = "pplEditor";
        objEditor.AllowEmpty = false;
        objEditor.SelectionSet = "User,SecGroup,SPGroup";
        objEditor.MultiSelect = false;
        Panel1.Controls.Add(objEditor);
    }
    private string GetAccountName()
    {
        string strAccountName = String.Empty;
        for (int i = 0; i < objEditor.ResolvedEntities.Count; i++)
        {
            PickerEntity objEntity = (PickerEntity)objEditor.ResolvedEntities[i];
            SPUserInfo objInfo = new SPUserInfo();
            objInfo.LoginName = objEntity.Key;
            strAccountName = objInfo.LoginName;
            // to return a sharepoint people group formatted string use...
            // strAccountName = objEntity.EntityData["SPUserID"].ToString() + ";#" + objEntity.DisplayText.ToString();
        }
        return strAccountName;
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string strAccountName = GetAccountName();
        Label1.Text = "account name: " + strAccountName;
    }
}

using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.SharePoint;
using Microsoft.SharePoint.Utilities;
namespace UserNameListTester
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                SPSite siteCollection = new SPSite("http://localhost");
                SPWeb mySite = siteCollection.OpenWeb("/");
                SPListItemCollection listItems = mySite.Lists["UserNameListTester"].Items;
                Console.WriteLine("Read in the existing values****************************");
                for (int i = 0; i < listItems.Count; i++)
                {
                    SPListItem item = listItems[i];
                    Console.WriteLine(item["PersonOrGroupColumn"].ToString());
                }
                Console.WriteLine("Writing new values**************************************");
                for (int i = 0; i < listItems.Count; i++)
                {
                    SPListItem listItem = listItems[i];
                    //listItem["Created By"] = mySite.Users["domain\\cornwell"];
                    // get the current logged on user
                    string currentUser = System.Security.Principal.WindowsIdentity.GetCurrent().Name.ToString();
                    listItem["Created By"] = mySite.Users[currentUser];
                    listItem.Update();
                }
                Console.WriteLine("Read out changed values*******************************");
                for (int i = 0; i < listItems.Count; i++)
                {
                    SPListItem item = listItems[i];
                    Console.WriteLine(item["PersonOrGroupColumn"].ToString());
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine("Shit: " + ex.Message.ToString() + ex.Source.ToString() + ex.ToString());
            }
            finally
            {
                string foo = Console.ReadLine();
            }
        }
    }
}