You never know when you might have a brilliant idea, or meet someone you've not seen for years and need to write down their email address. You could see a great price on something when you're out and about and want to check the price online later.
I mostly use my notebook for writing down article ideas and keeping track of my to-do list.
My personal favorite is my Moleskine Small Ruled Notebook. It fits neatly into my purse or my pocket and I get so much great use out of it. It also has a handy little pocket inside where I tuck business cards that people have handed me so I can get them safely home to file them away.
Amazon has the best price on these, but you can also find them in most bookstores.
Check out these other Saturday Handy Tips:
Overcoming Writers Block