Tcl

Wikipedia: Tcl wiki; Tk wiki
Tcl DeveloperSite
Tclers Wiki
Tcl/Tk at SourceForge

Tk Docs

Tcl/Tk Cookbook
C++/Tcl: A C++ library for interoperability between C++ and Tcl.
C++/Tk: A complete C++ interface to the Tk GUI toolkit.

Software Version Control

Subversion:
subversion(wiki)

Concurrent Version System
cvs(wiki)
The CVS Manual - Version Management with CVS

Git
Git - Fast Version Control System

Open Source Software

GNU:
GNU
GNU-FTP
GNU Manual
Free Software Foundation
Free Software Directory

Gnulib - The GNU Portability Library
GNU Libtool - The GNU Portable Library Tool
GNU libtool is a generic library support script. Libtool hides the complexity of using shared libraries behind a consistent, portable interface.

sourceware.org: hosting some free software including GCC, GDB, etc
GNU documentation tree from Utah University

IEEE Std The Open Group
This standard defines a standard operating system interface and environment, including a command interpretere (or "shell"), and common utility programs to support applications portability at the source code level.

Open POSIX Test Suite: A GPL Open Source Project

FOSSLLC: Free and Open Source Software Learning Center

Industry community:
IBM-Open Source
Intel-Software Network
google-code

Free Software Project Hoster:
Savannah; Launchpad; SourceForge; ohloh; freshmeat; github;

Opensource code search engine: Koders

GUI Toolkit

Cross Platform Widget Toolkit:
Wiki: Widget toolkit
Wiki: List of widget toolkit

Qt:
Qt

Qt Centre - The Ultimate Qt Community site
Qt Apps

Qt:

GTK+

Gtkmm:
gtkmm
Glade User Interface Builder

wxWidgets:

wxWidgets