No one will ever see your freewriting unless you want it seen. That makes it okay to be really inane, which cuts down on time spent sitting and tapping the pen you're not actually using. This freedom to write (see how that works?) often leads to breakthroughs, and I highly recommend it.
My freewriting mostly falls into two categories:
a) writing through a particular problem in the manuscript or question about what will happen next. (Often begins, "Hokay. So.")
b) having a character answer a writing prompt about anything so I can get to know the character and the novel's world a little better.
Today was a Category B day, since I wasn't trying to figure out anything in particular. I snagged a Monday Morning Warm-Up (Friday, Schmiday) (prompts are at the end of each post) and set out to complete the sentence, "I love my library because..." from my main character's point of view.
This particular prompt put my bookselling brain to work as well. What has my main character read lately? This information will probably never appear in the text, but it turns out she just finished James and the Giant Peach, and she loved it while she was reading it. That part was easy. The hard part was imagining how she feels about the book now; her current conflicts cast a few of my most beloved poems/songs from the novel in a completely new light. That taught me a bit about reader response and a lot about how the things my MC is going through affect the way she sees everything.
So what are your characters reading, and how do they feel about it? (By "your characters," I mean characters you've created, and I also mean characters you've read about and made your own. You don't have to be writing a novel to play this game.)