Labels are used a lot in GTK, and are relatively simple.
The Arrow widget draws an arrowhead, facing in a number of possible directions and having a number of possible styles.
The little text strings that pop up when you leave your pointer over a button or other widget for a few seconds.
Progress bars are used to show the status of an operation.
The Dialog widget is very simple, and is actually just a window with a few things pre-packed into it for you.
Ruler widgets are used to indicate the location of the mouse pointer in a given window.
Statusbars are simple widgets used to display a text message.
The Entry widget allows text to be typed and displayed in a single line text box.
The Spin Button widget is generally used to allow the user to select a value from a range of numeric values.
The combo box is another fairly simple widget that is really just a collection of other widgets.
The Calendar widget is an effective way to display and retrieve monthly date related information.
The color selection widget is, not surprisingly, a widget for interactive selection of colors.
The file selection widget is a quick and simple way to display a File dialog box.