#7 - Bone Folder / Scoring Board
When you're working with paper a lot, particularly with invitations or greeting cards, a bone folder is a must! It will help you fold and crease paper so perfectly, you'll wonder how you ever lived without one. It's not entirely necessary with thinner paper stocks, like printing paper or even scrapbook paper, but when you're working with cardstock or anything thicker than 80lb, you can see a huge difference.
The way the bone folder works is that you use the point (and a ruler) to create a score mark, and then you use the smooth sides of the bone folder to help fold the page down.
I have the Martha Stewart scoring board as well, and I think it's worth the investment if you're planning on scoring in large quantities. It has pre-made indentations, so you just line up your paper and it can score super straight lines without a ruler.
Personally, my method of choice is #5. I like the look of the crisp, clean inside and the ease and convenience of the score board. #2 is a close second, but that requires a cutting mat and a ruler.