Maybe I'm a bit more focused than other people, but when I get a new idea, I do not immediately want to put it into my current writing projects. What I actually do is to write the idea down in my notebook and possibly use it later--preferably in something else. So last night when I got hit with an idea while reading a book about organizing (yes, I know--dull reading), that's just what I did.
It was rather strange because the first thing that came to mind was the title and then a plot device and then finally the main character. However, it will have to take a fair amount of developing before it will become remotely doable. So right now, it's just that, an intriguing idea.
I do that too. Even if it doesn't look like it will work into my current story, there is always future stories. I find if I don't write it down, I am likley to forget.