Daniel has announced his "Holly Widgets" for Gtk#. A collection of useful Gkt# widgets. The widgets also support MonoDevelop/Stetic integration.
The code is available from http://code.google.com/p/holly-gtk-widgets.
Date and time picker.
Regular expression validating entries.
IP address entry.
Combo box with file system navigation.
Simplified API wrappers:
A simple combobox.
A simple list (avoids TreeView binding, and supports Measure/Draw on a per-item basis).
Simplified tree view API.
Combo box with tree view.
Daniel also has nice samples and documentation for all of his new widgets.