Shift + mouse wheel horizontal scrolling

In your designer file, you'll need to manually add a MouseWheel event delegate.

this.richTextBox.MouseWheel += new System.Windows.Forms.MouseEventHandler(this.RichTextBox_MouseWheel);

Then, in your code behind, you can add the following.

private const int WM_SCROLL = 276; // Horizontal scroll 
private const int SB_LINELEFT = 0; // Scrolls one cell left 
private const int SB_LINERIGHT = 1; // Scrolls one line right

[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, IntPtr lParam); 

private void RichTextBox_MouseWheel(object sender, MouseEventArgs e)
{
    if (ModifierKeys == Keys.Shift)
    {
        var direction = e.Delta > 0 ? SB_LINELEFT : SB_LINERIGHT;

        SendMessage(this.richTextBox.Handle, WM_SCROLL, (IntPtr)direction, IntPtr.Zero);
    }
}

For more information on the const values, see the following SO: How do I programmatically scroll a winforms datagridview control?

UPDATE

Use Alvin's solution if possible. It's way better.

If you are creating your own control derived from UserControl or ScrollControl or Form, you can use this simple solution:

protected override void OnMouseWheel(MouseEventArgs e)
{
    if (this.VScroll && (Control.ModifierKeys & Keys.Shift) == Keys.Shift)
    {
        this.VScroll = false;
        base.OnMouseWheel(e);
        this.VScroll = true;
    }
    else
    {
        base.OnMouseWheel(e);
    }
}

Explanation

If a control has AutoScroll and is displaying scrollbars, when you scroll the mouse wheel you will get the following behaviour:

1. If no scrollbars are enabled, it does nothing.
2. If only vertical scrollbar is enabled, it scrolls vertical scrollbar.
3. If only horizontal scrollbar is enabled, it scrolls horizontal scrollbar.
4. If both vertical and horizontal scrollbars are enabled, it scrolls vertical scrollbar.

Noticing this behaviour, I figured out this hack to override OnMouseWheel of the control, then if the vertical scrollbar is enabled and Shift is held down, it disables the vertical scrollbar before calling base.OnMouseWheel. This will trick the control in scrolling the horizontal scrollbar (behaviour 3 as shown above).