"I Wrote a Good Passage and Now I Don't What to Do with it."

Sure. If nothing else comes along I could fit it into, I'd try to build a new story around and from the passage itself. At the very least wring a short story out of it.

I have idea fragments all the time, some written as outline, some as a scene. I collect them. Every so often I look at some of them. If I start seeing an overall theme on some - e.g., set in NY - then I might call a folder "set in NY". Anything with a NY setting would be dropped into that folder.
If there are other themes, there will be other folders and one idea or scene fragment might make multiple appearances.

Helps me keep track of things. Every so often can look at a cluster in a folder and go "ooh, now if I added this one to that one, then the next step in the story would be...." and all of a sudden you are 20,000 words into a new book.

I have these problem sometimes. I think it's what I addressed in "Kill your babies?". I either try to weave them into the story somehow so they make sense, or I cut them with a heavy heart, but I save them for future use. You know, sushi is great, but that's no reason to force it on top of your chocolate cake just to have it on the table. Sometimes the great thing doesn't fit.

This sort of thing is why I always carry a notebook with me (even if it's a metaphorical notebook in the form of Evernote or something). You never know when you're going to think of a paragraph -- or even just a sentence.

And you never know when that little bit of writing is going to find a home in a current project or turn into something all on its own. I hold on to all of this stuff and try to remember to back through my bits and pieces on a regular basis to see what resonates.

If it doesn't fit in, I don't write it. I don't write random passages, random scenes, or even random sentences. That sounds self-defeating. The only reason I write any sentence is because I need that sentence to continue a particular story.

This happens to me a lot. I'll have a vivid idea for a particular scene, but won't know where I want it placed yet. Usually, I just write it up, put it in a second word document and wait for a place where it'll fit.

My current WIP is the first in a trilogy. I've written a scene that I think will take place half-way through the second book.

Yeah I do this a lot, usually it just sits there, maybe getting a bit more developed if it's lucky, although recently I've tried to slip them into my WIPs by moulding them to my existing characters. Only one of them has ever been anything more than just 'a scene' and that's the one that inspired my current WIP, becoming my first chapter and introducing one of my two POV MCS as well as setting up a major plot point. I guess it just depends on what the scene is, but I don't think writers should ever throw anything away, so it's good that you're keeping them

My strategy is similar to oceansoul's. Whenever I get an idea for an unattached scene, I make a sketch of it in a Word document that I keep expressly for this kind of thing. But I'll also make a note of the scene and where approximately I expect it to fit in my chapter-by-chapter outline. This "outline" is really just a really rough one- or two-sentence sketch of what I expect to be happening at some given future point of the story. It frequently ends up changing, but at least by referencing the unattached scene there, I'll remember that I have it and that I'm working toward it.

I have a file of Word documents with little bits and pieces that hit me out of the blue (or the middle of the night - damn you, brain!). I've developed a couple of books around nothing more than a little blurb or two. Others are still sitting there, waiting for me to do something with them. If I never use all these pieces, though, I won't consider it a loss. I think it's just good practice to get those ideas and thoughts down in writing, since I never know which one might be useful.