The D&D System
There are several competing systems of alignment, mostly inspired by Moorcock's Law/Chaos dichotomy.
The Original D&D scale is single axis, 3 position: Lawful, Neutral, Chaotic
The AD&D scale is two axis: Law vs Chaos, and Good vs Evil. Each is 3 position, with neutral in between. It was inherited into D&D3.0/3.5. It also was inherited by Hackmaster 4th Ed.
The Warhammer Scale is single axis, 5 position: Lawful, Good, Neutral, Evil, Chaotic. There is a note that the difference between Lawful and Chaotic is less than it would seem from the linearity of the scale. Bard Games' Arcanum uses the same scale, but calls lawful "lawful good" and chaotic "chaotic evil."
D&D 4E seems to have adopted the reduced 5 point scale from the center and corners of the AD&D system: Chaotic Good, Chaotic Evil, Lawful Good, Lawful Evil, and Neutral.
These can all be tracked by listing the axis, and assigning ranges to each item. Characters start in the middle of their chosen alignment's range(s), and if they take acts that are not suitable for their alignment, they move in where they are at on the grid.
This form of tracking was used in one AD&D supplement. It's also standard for Hackmaster.
It's not good for measuring the power, but is excellent for encouraging players to avoid out-of-alignment play.
Psychology Scale - Meyers-Briggs Type Inventory
Reich Star used the Meyers-Briggs scale. It's 8 axis, 2 positions each (I/E, N/S, T/F, P/J). I've read of several others doing likewise, but don't recall which. Unlike the previously mentioned ones, this one avoids matching closely to established moral norms, tho' ENFJ tends to fit most maniacal villains... and quite a few friendly people, as well.
Note that the Meyers-Briggs is used in Psychology, and people tend to vary over time. Tracking it for game purposes is pretty easy, but again, assign a point range on each of the 4 scales. (In fact, raw scores are numerical, and there is effectively a neutral area in each... I, personally, test out in the middle on all of them, usually on the ENTJ side. One particular run, I tested out as INFP... by 1-2 points of 16 on that form of the assessment.
Link to the Myers & Briggs Foundation Home page.
Palladium Books generally uses a 7 alignment system, but it's distinctly different from the 2-axis AD&D scale.
• Good Alignments: Principled, Scrupulous
• Selfish Alignments: Unprincipled, Anarchist
• Evil Alignments: Aberrant, Miscreant, Diabolic
I seem to recall at least one more, but what I've got to hand (Shadow Chronicles) only shows those 7. Each of them has a list of over 10 discrete behavior items. Tracking is pretty much a pain, but it's doable, because the list is pretty concrete. One could assemble similar lists for the D&D alignments.
Palladium uses a related but different system in Revised Recon:
Idealistic, Idealistic-Pacifist, Opportunist, Opportunist-Righteous, Opportunist-Karmic, Malignant, Malignant-Psychotic. The list of expected behaviors is short but broad. Malignant is essentially the ultimate expression of selfishness to the point of becoming assinine, and malignant-psychotic is worse. This does make tracking easier... but more subjective.
Pendragon is a 13 axis system, with each axis rated numerically. Each axis has two scores which, normally, total out to 20. It is used extensively in the adventures, and if you opt to act against it, you may wind up changing the scores by a point.
Pairings/axises: Chaste/Lustful, Energetic/Lazy, Forgiving/Vengeful, Generous/Selfish, Honest/Decietful, Just/Arbitrary, Merciful/Cruel, Modest/Proud, Pious/Worldly, Prudent/Reckless, Temperate/Indulgent, Trusting/Suspicious, Valourous/Cowardly
Inherently, the rules encourage and require tracking.
Moreover, each religion in the game marks 6 of the traits as important, using the ratings to determine if the character qualifies for a bonus. (They all have to be 16+ to qualify.) Magicians (which appear only in KAP 4E) use the sum of their religious traits to limit their magical abilities.
Several other games use reduced versions of this system, usually with 3-4 axis designs, or with no specific opposed pairs required, each player picking which scales his/her character is tracked upon.