I often compose snippets of writing in an unconventional text box: Alfred, my application launcher. If I have a sentence in mind, I can just hit Command-Space and start typing, and then copy it to the appropriate place after (Notational Velocity, a document, an email, Twitter, etc.). It removes another barrier to getting thoughts down. It’s also an easy way to compose a sentence from multiple sources, as I can quickly jump between them while Alfred remains just two taps away (but if Alfred is set to forget the last query after five minutes, be sure to copy often in case you stay long reading one source. I also use Jumpcut, which means I copy indiscriminately). Using an application launcher as a text input tool is especially useful when hand-copying text from a source like Google Books. It’s always available when needed, but stays perfectly out of the way, even when visible. I can’t think of a faster way to just start writing.
Of course, this technique works with any application launcher (as long as there’s no limit on the length of input), Alfred is merely what I’m currently using. And perhaps the large size of the input box is conducive to writing.