In addition to my obsessive love of books...I also love notebooks. I have kept a journal (although sometimes less frequently) since I was a teenager. There have been ebbs and flows, but over the past 8-9 years it has been more frequent. I find it cathartic.
I also like reading about notebooks, and what other people put in their notebooks...weird I know. I personally like moleskines, but I like notebooks and journals of all shapes, sizes, and designs. Check out the websites molskinerie and notebookism.
Here is a cool blog post on one person's use of notebooks.
I use my notebooks mostly for my personal journaling. I tried putting writing ideas in them, but that didn't work out well. I would start a notebook just for ideas and then put only a few down. I have also tried all that doodling and cool looking sketches and charts, but that is just not me...so I mostly write my personal stuff in them. Online tools and my blog have become the place for me to write the other more public stuff.
How do you use your notebooks?