As the name suggests, gtk2panel is a panel heavily based on the GTK+2 stack. The primary target of this project is to provide a panel that reuses whatever available in the obvious way, such as subclassing GtkWidget instead of implementing applets (whatever they are) or by using a GtkBuilder file instead of defining a new format to customize the panel.
To download the latest release, please visit the SourceForge project page. Historic releases are also availables and can be accessed from the same link.
This NRSI approach gives some additional advantages for free: you can use common tools in uncommon ways. For example you can embed a GtkHSV color wheel in your panel if you fill the need or put some shortcut icons in a GtkHandleBox, so you can drag them outside the panel window (this one is quite funny although there may be some tons of usability concerns). But above all, the main advantage is you can use glade to design your own panel (it still has some interface drawbacks though).