A form that can't be clicked.

TransparentUnclickableForm.cs

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    const int GWL_EXSTYLE = -20;
    const int WS_EX_TRANSPARENT = 0x20;

    [DllImport("user32.dll", CharSet=CharSet.Auto)]
    extern static int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    extern static int SetWindowLong(IntPtr hWnd, int nIndex, int nStyle);

    private void Form1_Load(object sender, EventArgs e)
    {
        var style = GetWindowLong(this.Handle, GWL_EXSTYLE);
        var newStyle = style | WS_EX_TRANSPARENT;

        SetWindowLong(this.Handle, GWL_EXSTYLE, newStyle);
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        this.BringToFront();
    }
}