What kind of trophy system is the best?

In Overgrowth, we are going to have some sort of achievement / trophy system. This is necessary for our Steam release, but we're going to take it a step further and integrate it into Overgrowth itself so Mac, Linux, and non-Steam Windows players can all have an awesome trophy system.

Iiro Jppinen made us this hot trophy icon

I have mixed feelings about achievements. On one hand, they are a common tactic to create the illusion of accomplishment without adding any real "meat" to your game. On the other hand, they have become standard in the industry, and most people (myself included) like them. The achievement meta-game is ubiquitous and practically required these days, so it's kind of a no-brainer.

I feel like a lot of games simply tack on achievements at the last minute because Microsoft or Valve requires them to. For instance, Duels of the Planeswalker has 12 achievements, most of which are things like "Win a game" or "Complete a challenge". These achievements are not really noteworthy -- they don't really commemorate a cool event or make you particularly proud.

As much as I'd like to bash Duels for having relatively boring achievements, I still enjoyed receiving them as I played through. The feeling of accomplishment is powerful, and even with the most basic of achievements, I think there is still a lot of value in them. I think that Armor Games' Achievement Unlocked demonstrates this very effectively. The actual game is amazingly crappy, but the meta-game that the achievements create is genuinely fun.

With a little foresight, I think we can do better; both in the quality of achievements and the achievement platform itself. I'd like to start some discussion about achievements before we begin development of our official achievement system.

A more realistic Overgrowth trophy

What are some of the best achievements you've seen in other games? Which system (Steam achievements, Xbox achievements, PS3s trophies, etc.) is the best?