Of my current projects, one sprung up simply because I have a deadline. I started out knowing I wanted alliterative poetry and wrote a bunch of garbage until I had a few things I thought were salvageable, then worked on those. It's now semi-coherent but still in need of editing.
Another, China, was spawned in a moment of intense feeling that I wanted to excise from myself by putting it on paper. It still serves essentially the same purpose, but has grown, of necessity, to have characters and overarching plot.
The werewolf romance was born when I was reading yet another of them, and decided that I should write one, to justify all of my reading of them as 'research.' Then I started diving into real research - legends, actual behavior and statistics for mundane wolves, population maps - and that shaped a lot of the beginnings of the story, since I wanted to be able to portray a lot of things missing from most of the genre in terms of the relationship between werewolves and real wolves (as in, have werewolves act more like wolves than macho men with 'roid-rage, much as I love macho men).
The Zimmerverse was born from a series of interconnected short stories I might eventually edit. It started as simply a platform for fairy tale retellings, and then I fell in love with a world in which this sort of thing could happen. Over the course of more stories, it unfolded further, until it became the world of Alexander, demanding a novel to explore it.
The collaborations were spawned in conversations with close friends who are also fellow writers, a cascading stream of 'what if's that ended up on a page.
Most blog posts start similarly, from a discussion. Sometimes they'll be sparked by reading other blog posts or by my own reaction to reading other things. They're part of a wider discourse on writing and the publishing industry, so they're much more open-ended, and not as self-contained.