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