"A sense of achievement without the capital ‘A’ is a better reward than anything Microsoft’s Funpoints can throw at you, and the very best Achievements are ones which reward actual achievement. When a player does something he or she know is awesome and the game agrees with them , it only makes the sense of achievement greater. Success in a game is better when you have an audience – in the absence of nearby friends, Achievements should be your audience."

There's an achievement in HL2:Episode 2 which involves carrying a little gnome through the entire game and doing a thing with him. A lovely writeup here.