Scid
Welcome to the website for Scid (Shane's Chess Information Database), a chess database application for Windows, Linux and Mac OS X operating systems.
With Scid you can maintain a database of chess games, search games by many criteria, view graphical trends, and produce printable reports on players and openings. You can also analyze games with the Xboard or UCI compatible chess program, play online on FICS, and even use Scid to study endings with endgame tablebases.
Resources
There are a lot of places on the internet where you can download chess databases.
Also many chess engines exists that can be used with Scid.
Check out the list on the wiki.
Download
Scid is released under the GPL licence and works on Linux, Windows and Mac OS X. Sources and precompiled binaries are available on sourceforge.
Franz Nagl provide additional downloads for maintaining high-quality databases: spellings, ratings and photos of chess players.
Current stable version is 4.4
Current development version is 4.5
Help
Tutorials and help pages are a little outdated.
Even if still incomplete you can check out the new wiki pages, and help with writing docs pages are welcomed.