The object of the game is very simple: post a legitimate superpower (something a person would not normally be able to do) that would be useless and attempt to make it more useless than the previous one.

Here we go:

Being able to transform your left index finger into a cardboard spork only once. Once you transform your finger, it never can change back.

I'll repost and explain, then.

The power to place a quark which you are observing into a superstate.

Why it's useless:
1.) No provision made to observe quarks.
2.) Since you are observing the quark, the superstate will collapse immediately upon being instituted because you are observing it.

If the ability is interpreted as allowing you to observe quarks, then provision two still holds. It could probably be argued that being able to observe quarks is a useful superpower, in very specific instances, but for general use this power would be inapplicable.

The ability to unhook a single velcro hook at will once per week.

Velcro is made of thousands of hooks that loosely lock together when closed; you can see how this is useless.

Hmmm...

The ability to play FATAL without damaging one's mental health.

Explanation shouldn't really be necessary, but if needed:
1.) You might be able to play it. Your friends won't be able to...
2.) You would still be playing FATAL...

As another one to continue the roll,
The ability to easily create programs in Malbolge.

Look up Malbolge(programming language), and you will know why this is here.

O.K, I got one.
The ability to convince my wife she's wrong.

useless because:
1) Even if she's wrong, she's right.
2) Even if she's wrong, she's less wrong than you are.

Sorry about that. Rant over.
I have a real one that you actually see in some movies/comics/whatever. The ability to make yourself "insubstantial" so bullets, fists, explosions etc. go straight through you.

Think about this one, there are two ways it could work:
1) Only matter goes through you, you're still affected by forces such as gravity etc.
2) Nothing affects you, the laws of the universe dont apply.

In either case.
1) You fall straight through the earth, pulled by gravity to the core where you remain eternally.
2) You are released from the loving caress of the earths gravitational field, to float upwards as the planets orbit pulls it away from you, leaving you floating in empty space, where you remain eternally.

Fun either way, huh?

EDIT- I thought about this, and actually in the first scenario you wouldnt stop at the core, your inertia would mean you would get to exactly the same distance from the core as you started in the opposite direction, i.e. the exact opposite side of the world, then start going back again, ad infinitum.

Most insubstantial types also get flight powers of some form or other. So not nearly so useless.

Here's one for you: a superhuman sense of taste. It might have a use once in a while. But totally not worth what you're missing out on.

Perhaps the ability to ignite oneself? Without fireproofing, that would be fairly hard to use...

Super Drowning Skills probably don't count...

Teleportation if you can't change your velocity on exit (assuming that you can hit your target in the first place...).

Getting a dog tail. Hounds use their tail to help their balance when running. But since you are walking (and running) upright, it wouldn't really help. In addition, you would have to refit all your pants to either fit it inside, or let it go out by some hole. And you would be wagging it every time you're happy, so it would be harder to lie.

The ability to have your presence announced by a blast from an exceedingly loud siren (without having to have sirens present).

Sure, you could probably startle someone or throw a military base into disarray, but it wouldn't be all that useful.

Perhaps slightly more useful, and not getting you arrested every time you use it, would be the ability to play a piece of music at a volume of your choice.