Xfce Embed Plugin enables the embedding of arbitrary application windows into the Xfce panel. The window is resized into the panel space available, and the associated program can be automatically launched if it is not open. Example uses include embedding an instant messaging buddy list, a mail client's new mail ticker, a simple media application, or a fancy clock or timer. Combining with Xfce's ability to auto-hide panels can make this very convenient.
Panel Fix is a program that allows you to save and restore the positions and states of GNOME panel applets. It can be useful if your panel applets get rearranged against your will. When saving applet configuration, the program will also save some additional information for easier manual configuration.
Compa is a GNOME panel applet which can display the output of any system command on the panel. It can also display the output of a different command in its tooltip, and launch a different command when clicked. The text style can be modified using the Text Attribute Markup Language.
The primary aim of the Gtk+2 panel project is to provide a panel that reuses whatever is available (if it is not overkill) in the obvious way, such as subclassing GtkWidget instead of implementing applets, or by using a GtkBuilder file instead of defining a new format to customize the panel. This approach gives some additional advantages for free: you can use common tools in uncommon ways. Above all, you can use Glade to design your own panel. All the dependencies apart from GTK+ are (and hopefully will be) optional.