Pens, blank notebooks--but now, whenever I see a Moleskine notebook at Borders I repeat to myself "I already have more than I need" until the urge to buy it goes down.
Scrapbooking papers/pens and other stuff--I haven't been as successful in stamping out the urge to buy more paper or stuff, although I didn't buy anything for a few months. The urge came right back as soon as I actively got back into it.
Totebags/purses--this is not as severe as many people's. I finally found an REI bag that was exactly the right size, carried most of what I like to carry around with me (any larger and I'd be a RV turtle) and that has turned my impulses away from other purses.
I used to have a book obsession when I was younger. I'd only buy books that I was sure to reread, that were my favorites. My collection was enormous. Two basement floods in two separate houses reduced my books by a huge number. I think that was the start of my holding back on books. Lately I go into a bookstore and find myself realizing that I don't buy fiction like I used to, but I *will* buy nonfiction (about scrapbooking!) once in a while. I've even sold some books to Powell Books. I have my eye on other books I bought when I was a Jr. High student for my future children. They aren't interested in them at all. Oh well. Off to Powells they go, if books are sellable.