ChessX is an Open Source chess database. With ChessX you can operate on your collection of chess games in many ways:
browse, edit, add, organize, analyze, etc.

ChessX is cross-platform. It works on Windows, Linux and Mac OS X.
Current Features

Multi-platform. Supports Windows, Linux and Mac OS X
Load and save PGN files
Work with multiple databases simultaneously
Browse games, including variations
Enter moves, variations, comments
Play games in training mode or play out games against an engine
Setup board, copy/paste FEN, merge games
Search header (click on columns in GameList header)
Display opening tree for current position
Analyze using UCI and Winboard/Xboard Chess engines
Observe and play games on FICS

Feature fantasy:

Perform advanced search (board, header, material)
Store games in native binary format
Fix spelling in databases
Play/train against engine
Calculate tournament tables
Calculate ELO performance