WPF Elements provides two components that are used to compose other elements into larger controls.
The DropDownEditBox control provides a ComboBox-like experience without the restriction to a specific list of items that the ComboBox imposes. You use the DropDownEditBox to compose two other components: a compact "edit box" view and an expanded "selection" view. For example, a colour editor could display a name or RGB value in its "edit" view, and a colour wheel or sliders in its "selection" view.
The ProportionalStackPanel adds proportional layout functionality, similar to star-sizing in a Grid, to a StackPanel. This is useful when using the panel as the ItemsPanelTemplate of an ItemsControl, where you can't use a Grid because you don't know how many items there will be or how much space needs to be allocated to each.