Plumbob

A PlumbBob is used in most titles of the Sims series to identify the character selected. It is also used to show the mood of playable Sims.

According to the "Atlas's Burden" statue in The Sims 3, the PlumbBob is a tangible representation of the "celestial weight" (the player) controlling the Sim's world.

The Sims
In The Sims, the PlumbBob is a fairly accurate reflection of the Sim's mood. If the Sim is in a very good mood, the PlumbBob will be a bright, deep green. As the Sim's mood gets lower, the green will become paler until it becomes a sort of neutral color. A Sim in a bad mood will have a red PlumbBob, and the red will get brighter and deeper as the Sim's mood gets worse.

If the Sim has a date in Downtown or Old Town, or on Vacation Island, the date will have a small blue PlumbBob, which will reflect their mood. The deeper the blue, the better their mood.

(Some of the clips in the intro movie for The Sims show a larger PlumbBob that is shaped differently from the one that appeared in the game.)

The Sims 2
In The Sims 2, the PlumbBob is white if the Sim is in platinum aspiration, which produces a very high mood regardless of the Sim's motives. Otherwise, it is bright green if the Sim's mood is high, and orange or bright red if it is low. If the Sim's mood is somewhere in between, it will be a shade of green-yellow reflecting the approximate level.

Perma-platinum can be achieved through completing the Sim's Lifetime Want if the player has University through Bon Voyage, however if the player has FreeTime perma-platinum can be reached through completing the life achievement bar.

Also in The Sims 2, Sims who belong to the same active group as the selected Sim will have a smaller plumbbob over their head. This PlumbBob is blue, irrespective of mood.

The Sims 3
In The Sims 3 with The Sims 3 World Adventures the PlumbBob is green if everything's right, yellow if the Sim isn't good and red if the Sim is in bad humour and has bad moodlet or low needs. There is a new meaning of the yellow colour introduced by World Adventures. It is the subject of speculation, some think it means the Sim is in an "adventurous" mood, but most think that it is just the mood between green and red, as in The Sims 2.

Controlling device?
In the Sims 2 PSP storyline, which it seems to be the events after Sims 2 (become the latest story, since The Sims 3 is before The Sims) the player sim will encounter Dr. Dominic Newlow aka Dr. Dominion (the antagonist character) created a device that can create the "Green Diamond" to control other people including the Player sim. The ending of the game breaks the fourth wall, as he reveals that the "Green Diamond" is a mind control device used by an "Outside Force" (Player). As the normal gameplay other than the storyline, there doesn't seems to be any PlumbBob at the top of the Player sim, but still controlled by the player. It seems that the player control the sims using the PlumbBob (Selected sim will have a PlumbBob on their head and become controllable) which become the game mechanics until revealed at The Sims 2 (PSP).