I guarantee you everyone's had one at least once… (Well, maybe not *some* people)

Here's an easy one, since the 50th puzzle is still being solved.

I'm as tall as the largest mountain, but can fit in your hand.
I can be spread out like a fountain, slip through fingers like sand.

I can plague you for days, like an itch on the back of your neck.
Or I can astound and amaze, and give you one large paycheck.

What am I?

Hint:

I'm not physical.

• You created your $50$th puzzle? Congratulations! :) – Mr Pie Aug 1 '18 at 22:51
• Would the correct answer work any better when spoken than when written? – Brent Hackers Aug 2 '18 at 14:39

Is it

an Idea

I'm as tall as the largest mountain, but can fit in your hand.

some big ideas can be written in a piece of paper

I can be spread out like a fountain, slip through fingers like sand.

Some people spread their ideas by speech or writing
and, If you didn't pay attention to your idea, your idea might be forgotten (or stolen)

I can plague you for days, like an itch on the back of your neck.

"An idea is like a virus, resilient, highly contagious", Cobb (Inception, 2010)

Or I can astound and amaze, and give you one large paycheck.

Good and implemented idea will give you some rewards even money

• Sorry, this is my first chance to see this answer. This is correct. – Joe-You-Know Aug 2 '18 at 14:44

I'm as tall as the largest mountain, but can fit in your hand.

A tall tale can fit in a small book.

I can be spread out like a fountain, slip through fingers like sand.

Can spread wildly or easily lost.

I can plague you for days, like an itch on the back of your neck.

Cliff-hangers... I want to know how it ends.

Or I can astound and amaze, and give you one large paycheck.

People sell stories, some with shock and awe.

• This is great, but not quite what I had in mind. It certainly fits, though. – Joe-You-Know Aug 2 '18 at 15:00

Are you a

Computer?

I'm as tall as the largest mountain, but can fit in your hand.

Computers can have cloud backup and you can hold a laptop with one hand.

I can be spread out like a fountain, slip through fingers like sand.

Maybe the wifi network?

I can plague you for days, like an itch on the back of your neck.

I may spend to much time on my computer, lurking on puzzling.

Or I can astound and amaze, and give you one large paycheck.

You can be a programmer, or access your bank on the computer.

The title

Some people here use the computer while others use mobile.

And,

El-Guest said the first letters, IIIO, stand for "Intel Inside, Idiot Outside"

Are you a

An solved riddle?

I'm as tall as the largest mountain, but can fit in your hand.

There are some big questions.

I can be spread out like a fountain, slip through fingers like sand.

You may have an answer for some puzzles but other answers may slip you mind.

I can plague you for days, like an itch on the back of your neck.

Myabe the question has a bounty on it and you still cannot answer it.

Or I can astound and amaze, and give you one large paycheck.

You may find the answer to an awesome puzzle and maybe receive up votes and a bounty.

And,

Questions and answers are not physical. and some people on this site have not answered questions.

• And, my favourite part, rot13(Gur svefg yrggref, VVVB, fgnaq sbe "Vagry Vafvqr, Vqvbg Bhgfvqr"!) – El-Guest Aug 1 '18 at 16:45
• My pleasure! Great answer though, you were super quick today!! :) – El-Guest Aug 1 '18 at 16:49
• It's not this, though this is great. :D – Joe-You-Know Aug 1 '18 at 18:38
• It's not this either, though this is WAY closer to the actual answer. – Joe-You-Know Aug 2 '18 at 1:19
• @Joe-You-Know I would answer with this ro13(Pbhyq vg or na nafjre?), but I just want to check first. – QuantumTwinkie Aug 2 '18 at 3:27