This simple function returns the file name (i.e. default.aspx) of any page that it is called on.

VB.Net
Code:
Public Shared Function GetCurrentPage() As String
    Return System.IO.Path.GetFileName(HttpContext.Current.Request.Url.AbsolutePath).ToLower
End Function
C#
Code:
public static string GetCurrentPage() 
{ 
    return System.IO.Path.GetFileName(HttpContext.Current.Request.Url.AbsolutePath).ToLower; 
}