The ability to fart or burp on cue.

Actually, given the so-called "humor" from hollywood these days, you could make a rather successful career on that alone.

You'd think, but someone like that would probably be completely ignored by the media; likely to make it as far as congress, but still be considered 'unelectable' as a president. :P

The ability to differentiate between varying shades of colors, but only in the ultraviolet spectrum.

The ability to detect the presence of bugs within a 30 foot radius, but not their direction or distance.

How about a ring that allows the character to become massively powerful---like, Yoda on crack powerful---but is completely powerless against a certain color.
Let's say it's a primary color. Like...hmmm...how about yellow? So that any ill-tempered 4-year-old with a sharpened #2 pencil should be able to take the hero out.
Oh, wait...

Lol Green Lantern reference.

The ability to use the force, but only when your feet are frozen to the ceiling of a cave.

The ability to make dogs pee on command.

Bullet-proof left kidney.

The immunity to sleep-inducing substances, but only after two weeks of insomnia and taking care of spoilt little children.