How to Filter Navigation widget by the first letter of the page?
How to extend the Navigation widget to filter by the first letter of the page

1. Create a CustomNav.cs file in your project

2. Paste the following code in it:

public class CustomNav : LightNavigationControl
    {
        public string LetterFilter { get; set; }
        protected override void InitializeControls(GenericContainer container)
        {
            base.InitializeControls(container);

            if (this.SelectionMode == PageSelectionModes.CurrentPageChildren)
            {
                if (!string.IsNullOrEmpty(this.LetterFilter) && this.LetterFilter.Length == 1)
                {                   
                    var currentSiteMapDataSource = this.DataSource;
                    var node = SitefinitySiteMap.GetActualCurrentNode();
                    var allNodes = currentSiteMapDataSource.Provider.GetChildNodes(node);

                    var filteredNodes =
                        from n in allNodes.Cast<SiteMapNode>()
                        where n.Title.ToLower().StartsWith(this.LetterFilter.ToLower())
                        select n;

                    this.DataSource.Nodes = filteredNodes.ToList();
                }
                else
                {                    
                    var errLbl = new Label();
                    errLbl.Text = "You need to specify LetterFilter from Advanced settings and it should be 1 character only.";
                    this.Controls.Add(errLbl);
                }
            }
        }
    }

3. Build the project and then register this control in the toolboxes of Sitefinity as a custom widget:

Sitefinity Documentation: http://docs.sitefinity.com/register-a-new-widget-in-sitefinity-toolbox

4. Run the project in the browser and drop the newly registered control on a page.

5. Go to Advanced settings of the control and find the LetterFilter property which you can set to a single letter (for example A).

6. Save.