Writing Prompts

I noticed that after my last post or two, several people happened upon my blog while searching for "writing prompts." I got to wondering what prompts us to write. If you write for publication, there's the whole money, fame, food-on-the-table thing. But what prompts us to write on a deeper level?
Some published writers will call blogging or journaling cheap therapy. And maybe these kinds of writing are. Maybe that's not a bad thing. Writing is a form of communication. It can help internalize the positives of life and externalize the negatives. Shared writing creates community. Private writing facilitates the development of personal space and that ever important "sense of self."
Even with all the reasons I am motivated to write, it still takes a certain something to prompt me to write on any given day. The general motivations are not enough. They can be countered with the promise of a movie and a bubble bath. A prompt has to help me overcome all the reasons not to write. An external prompt like the Thursday Thirteen or Sunday Scribblings can help with some of those reasons. You know, the "I'm not creative. I don't know what to write" reasons. External prompts also come in the form of books and other resources with sentence starters and other creative material.
Believe it or not, prompts can also come from yourself. When I take a moment to write down something a coworker said, or make a note of something I saw that inspired me, that becomes a writing prompt. Routines are a writing prompt. I write three pages in my journal a day. Some of it makes its way into material that will be published or otherwise productive. Some of it is just a brain dump that helps me sleep. Both purposes serve as writing prompts.
Some prompts focus on inspiration for writing. Some focus on the responsibility to write. What writing prompts do you use?