Toolbar Icons is a collection of free toolbar icons. All icons are available as (Inkscape) SVG files. OMake build scripts that automatically generate PNG files of basically any size (using Inkscape) are included. PNG files in a few default sizes (16 x 16, 24 x 24, 32 x 32, and 256 x 256) are prepared in .tar.gz archives.
Flexible Localization is a .NET/Mono library for string-based user interface localization. It offers a hierarchical structure (which can be partially loaded) to organize the strings, as well as support for several independently-localized modules. Localization files are validated against a localization declaration which defines the actual strings that are expected to be in the localization files. The strings themselves can be parametrized and evaluated based on expressions (i.e., different strings can be returned by a localization, based on parameter values).
The Two-Layered GUI Toolkit is a library of GUI helper classes which is designed in a way to simplify development of separate interchangeable front-ends. For every supported dialog box type, there is a settings class (independent from a GUI toolkit) whose options can be used by one of the implementations for a specific GUI toolkit (bindings for Gtk#, WPF, and WinForms are provided, but custom ones can be added). That is, the dialog definition can be prepared by the application's core component and passed to the front-end, which uses it to instantiate the dialog box using whatever GUI toolkit it likes.
Tree Layout Helper is a .NET/Mono library that provides some utilities for handling tree structures. Most importantly, it supports the display of tree structures as two-dimensional graphics. By using ready-made interfaces, you can specify the size of your nodes, and the Tree Layout Helper library will take care of placing those nodes appropriately.