Getting the logged on user name using the client object model

If you want to use the SharePoint 2010 client object model you can use the code below:

ExecuteOrDelayUntilScriptLoaded(getWebUserData, "sp.js");
    var context = null;
    var web = null;
    var currentUser = null; 

        function getWebUserData() { 
        context = new SP.ClientContext.get_current();
        web = context.get_web();
        currentUser = web.get_currentUser();
        currentUser.retrieve();
        context.load(web);
        context.executeQueryAsync(Function.createDelegate(this, this.onSuccessMethod), 
             Function.createDelegate(this, this.onFailureMethod));
   }

    function onSuccessMethod(sender, args) {
        var userObject = web.get_currentUser();
        var UserTitle = userObject.get_title();
        alert(UserTitle);
        }

    function onFailureMethod(sender, args) {
        alert('request failed ');
    }


Here is a video and a simple way to test it.