I carry a Moleskine notebook with me pretty much everywhere I go. I'm on my fourth such notebook at the moment. I use them to jot down rough notes for stuff that might eventually make it into emails to friends, or posts in the Red Notebook or on my other website. I also stick stuff into them to make them a bit more interesting.
There are an awful lot of people out there doing really cool stuff in their Moleskine notebooks. I admire their work, and enjoy looking at the images they post on Flickr, but I have neither the desire nor the ability to produce anything quite as fancy in my own Moleskine.
However, just for a bit of inspiration, the other week, I took a few images produced by scientific geniuses, resized them to fit neatly into my Moleskine, printed and cut them out, and stuck them into my notebook. I'm quite pleased with (and inspired by) the results: