An "Idea Box" is a concept recommended by author Joan D. Vinge -- keep your ideas on 3x5 cards as they occur to you. Later, when you need an idea for a story or want concepts to flesh out a story, consult your Idea Box.
My own Idea Box is a file on my computer, but basically I do what Vinge suggests, and it's been a great resource. I'm up to 915 entries at this point. Most of these are very fragmentary, and maybe only useful to me. Others are close enough to full story outlines I want to keep them private. But I figured I'd share some of the rest here from time to time. Here's one...
Murder, She Thought
A Miss Marple-type character, who is inexplicably involved in murder mysteries several times a year, is unconsciously responsible for them. She has an uncontrollable telepathic gift that encourages those with a strong motive to kill to actually do so, and in convoluted ways that make for interesting puzzle-solving on her part. She would be horrified to learn that she is herself responsible, and will refuse to believe it. If confronted with the truth, she will deny everything... but her power will work overtime to silence the accuser.
(As a twist, maybe it turns out there are lots of people around whose minds can force others into a sort of genre mold -- soap opera, adventure, romance, academic/literary fiction, and so on. Our telepathic "detective" may have to be brought down by another telepath who lives in a world of romantic comedy...)