The WebCombo can deliver such functionality. But as it can render a set of columns in the drop down area (making it a heavier component), I wouldn't recommend it's use unless you have some scenario that requieres something like this. Using a normal asp control would be fine. If you really need (or want) to use the Infragistics WebCombo, you have to set some properties to make it look like a drop down. Some of them would be:

ultracmb1.DropDownLayout.DropdownWidth = Unit.Pixel(104);
ultracmb1.DropDownLayout.DropdownHeight = Unit.Pixel(70);
ultracmb1.DropDownLayout.ColHeadersVisible = Infragistics.WebUI.UltraWebGrid.ShowMarginInfo.No;
ultracmb1.DropDownLayout.RowSelectors = Infragistics.WebUI.UltraWebGrid.RowSelectors.No;