I had a need for a combobox that allowed multiselect and checkbox.  WebCombo was very close to what I needed, with some simple code i was able to achieve this functionality so i figured i would share with others who may be interested.

Steps:

Drop a webcombo on your form:

In page_load add the following code: ( I set properties in code for webcombo)

//create datasource

tbl.Columns.Add("Id", typeof(int));

tbl.Columns.Add("Text");

//just some adjusting of the height

//get a reference to the grid in the combo

//set some properties of the grid to act and look like a combo

grid.DisplayLayout.ClientSideEvents.CellClickHandler = "GridCellClickHandler";

grid.Columns.FromKey("Checked").CellStyle.BorderWidth = Unit.Pixel(0);

grid.DisplayLayout.ActivationObject.BorderStyle = BorderStyle.None;

grid.Columns.FromKey("Text").Width = Unit.Pixel(160);

//set clientside events Combo box Properties for look

WebCombo1.ClientSideEvents.BeforeCloseUp = "WebCombo1_BeforeCloseUp";

WebCombo1.DropDownLayout.RowSelectors = RowSelectors.No;

WebCombo1.DropDownLayout.ColHeadersVisible = ShowMarginInfo.No;

WebCombo1.DropDownLayout.FrameStyle.BackColor = System.Drawing.Color.Transparent;

//if set to false then selected values won't show in combo until you click another spot on the form

//prevents closing of combo while selecting multiple items 

//Cancels premature closing.

//Loop through grid and find all rows that are checked and concatenate the text.

var grid=combo.getGrid();

var displayText='';

var checked=row.getCell(1).getValue();

//Changes checkbox selection on any cell click in the row

cancelClose=true;

//Prevents dropdown from cloasing after clicking checkbox

var grid=combo.getGrid();

var displayText='';

var selectedText=new Array();

var row=grid.Rows.getRow(i);

bool isChecked = (bool)row.Cells.FromKey("Checked").Value;

 Here is what it looks like: