I've released two open-source Tcl/Tk packages that add some needed functionality to Tk in a cross-platform fashion.
The first package, TkWebView, provides a lightweight HTML viewer that can either be used as a standalone window or embedded in another widget. Tk is conspicuous among cross-platform GUI toolkits in lacking an easy-to-use HTML widget. TkWebView only supports basic HTML, not tables, CSS or other components of web browsers, but it still fills a void in Tcl/Tk. (An ambitious project to provide a modern HTML widget for Tk, TkHTML 3, appears to have stalled--and even when development was moving forward, it suffered from an extremely high level of complexity that made it nearly impossible to use in an application.) TkWebView could be used as a help viewer, a widget to display HTML from RSS feeds, and more.
The second package, SimpleDND, provides a lightweight mechanism for drag-and-drop within a Tk application. There are various methods to display drag-and-drop in Tk applications, but they are either limited (quick hacks specific to a particular application or widget), complex (such as BWidgets' drag-and-drop mechanism), or not fully cross-platform (TkDND is the most powerful DND extension out there, supporting drag-and-drop between different applications, but is not supported on the Mac). SimpleDND provides basic drag-and-drop visualization with a few commands, and such fills a void.