The GTASC (Great TrueAchievement Score Challenge) is run every year by planting42.
How it works: The contest runs for the entire calendar year. Each week some of the competing teams are eliminated from the team competition and some individuals are eliminated from the individual competition.
Eliminations: As a team, you try not be one of the lowest scoring teams of the period, based on their TrueAchievement score gains for that period. Similarly, as an individual, your goal is to stay above the elimination line. Those falling under the line go home empty-handed.
Important note: TrueAchievements only track achievements earned while you are connected to LIVE (aka online achievements). 'Hiding' - or appearing offline doesn't affect this as you are still connected to LIVE and receive timestamps for your achievements.
See the GTASC contest forum for more info.