Valve announced today that a new major feature will be included in Steam's next client update: Steam Cloud.
Steam Cloud will store players' save games and configuration online in a completely transparent manner. Half Life and Half Life 2 will be the first games to support Steam Cloud. Counter Strike will then follow by storing user key mapping. Left 4 Dead will ship with full Steam Cloud support.
There will be no quotas or limits on how much data a player can store per game.