You can never have enough...
1. Time. Why is it that there is always something else you should be doing? Something along the lines of "Hmm, I feel like making a card or doing a layout. Oh, but I need to hang the clothes, do the dishes, wash the floors, change the sheets, weed the garden, make the dinner, bath the kids," you get the idea. I'd like to have that remote in the movie Click so I could pause the rest of the world for a few hours each day.
2. Pens. I have to admit that even before I knew the wonderful world of papercrafts I loved stationery. Pens in different colours - gel pens, fountain pens, calligraphy pens, felt tip pens. And funny as this may sound, I write differently depending on the pen. Give me a felt tip pen and I tend to adopt a soft slightly cursive writing; give me a ball point pen and I write in a plain lowercase print. I often also go over my writing - especially on a scrapbook layout, just to make it darker and 99% of the time it is exactly over the previous letters. I've always loved handwriting, and my love of pens has come from that, and you can never have enough pens.
3. Stamps. If you had told me 5 years ago that I would become a bit of a stamp addict I would have probably thought you were talking about postage stamps. I love rubber stamps for many reasons, although I think the number one reason is versatility. They really are an investment and you need to consider how much you would use them. I tend to choose stamps that have more than one application, eg. background stamps that can be used on a range of card themes or layouts, or images that can be used for a range of themes. This way I know that I will be getting value for money and I will use it over and over again without repeating the same design. That said, you can still never have enough stamps.