IApplication.CurrentTool Property

This function makes the Identify tool the currently selected tool in ArcMap.

public void CurrentTool(IApplication app)
{
    ICommandBars documentBars = app.Document.CommandBars;
    UID cmdID = new UIDClass();
    cmdID.Value = "{B7FA188F-EBE3-11D0-87FE-080009EC732A}";
    ICommandItem cmdItem = documentBars.Find(cmdID, false, false);
    app.CurrentTool = cmdItem;
}

This Sub makes the Identify tool the currently selected tool in ArcMap.

Public Sub SetCurrentTool(IApplication app)
   Dim documentBars As ICommandBars = app.Document.CommandBars
   Dim cmdID As UID = New UIDClass
   cmdID.Value = "{B7FA188F-EBE3-11D0-87FE-080009EC732A}"

   Dim commandItem As ICommandItem = documentBars.Find(cmdID) 
   app.CurrentTool = commandItem 
End Sub