University of Pittsburgh (UPITT) Chess Archives
Good starting place for thousands of downloadable master-level chess games. No longer updated regularly.
GambitChess
Downloadable files with games from well known chess books. In Italian and English.
ChessliB
Downloadable game database organized by ECO opening code. Also has an endgame database.
Chessgames.com
Online database of historic chess games with a search interface. If you're new to game archives, see the Help page.
ChessMix.com
Database of recent games (called '10 Days' after its update frequency), calendar, and tools. By GM Ventzislav Inkiov for the serious player.
Portable Game Notation (PGN)
Downloading games is one of the most popular chess activities on the Internet. PGN makes it simple. The 'Tower of Babel' for chess stands tall.
PGN standard
The original Portable Game Notation standard in Web page format.
An Extension to the PGN Standard
Addresses some of the issues that the original standard missed.