Drop-Down Controls for .NET - https://www.brad-smith.info/blog/projects/dropdown-controls

Includes:

• GroupedComboBox - A ComboBox with grouping (group headings only, depth 0-1)
• ComboTreeBox - A ComboBox with a TreeView drop-down (infinite depth, supports node styles, images and checkboxes)
• Custom DataGridView column types - Allows GroupedComboBox and ComboTreeBox to be hosted in the DataGridView control
• DropDownSearchBox - A ComboBox with inline searching (search within the local list items or an external data source)

Requirements:

• .NET Framework 4.5+