Kitchen Gadgets. Yep, if there’s a left-handed widget that can only be used by virgins in the light of the full moon I’ll buy it. That I can’t use it is immaterial. Doo-Dads must be bought!
Cats.
Books.
CDs (and the subsequent MP3s stored on an external hard drive), especially Christmas music. Yes, I have Ricky Martin’s Ay Ay Ay It’s Christmas and many worse than that.
Shoes (especially ridiculously impractical heels)
Books about Japanese
Books in Japanese
Anything hot pink
Cool gadgets I read about on the Dope and immediately develop gadget lust for…
T shirts with goofy sayings on them
Amber jewelry
Anything involving a dragonfly
Fabric
plants and flowers
My husband
Pajamas. Even though I rarely actually wear pajamas to bed, I will buy set after set of them.
Shoes. The more impractical and the less likely I am to ever wear them, the better.
Books. As has been mentioned, I know they can be gotten from the library. I want none of that. I want to buy the book and dog-ear the pages and read it again and again and break the spine and have people who look at my library know that my books have had the life read out of them. If I destroy it, I can buy another copy.
Cat Toys. The cats could care less, they’d rather play with the box the toy came in.
TV Show Box Sets. I’ve bought sets for shows I’ve never even seen, just because I think I’ll like it. (In my own defense, I haven’t not liked a single one yet.)
Paranormal Romance Novels. I’m an intelligent, well-read, relatively sophisticated woman, but you give me a book about a time-traveling spunky chick who suddenly finds herself beset by an enormously handsome vampire while trying to solve the mysteries of the ancients, I’m so there.
Anything with a Superman logo on it. And I’ve got five different sets of Superman pajamas, actually. 
guitars, guitars, guitars, guitars, guitars. I loves me some gui-tars. I could easily spend all of my money on guitars.
let’s see what else…
good coffee, good german beer, good wine
kitchen gadgets and good culinary knives
DVDs, but i try to keep myself from buying crappy movies.
All types of music.
audophile products. right now I’m running an HK receiver through a set of klipsh speakers. My girlfiend just shakes her head and smiles…
Cookbooks
Dogs
Cool shoes
Kitchen stuff
I second a bunch of already-mentioned stuff (with nods to the original poster):[ul][li]Time-waster threads 
(Mynn)[/li][li]Free food, whether it’s good or not. (Big Bad Voodoo Lou)[/li][li]Animals. (Licentious Ectomorph)[/li][li]slang dicitionaries and books on word derivation (twickster)[/li][li]Places I haven’t been to yet (Robot Arm)[/li][li]Stationery stores. (StarvingButStrong)[/li][li]Salsa and chips. (Big Bad Voodoo Lou)[/li][li]Cold pasta salads. (Big Bad Voodoo Lou) (I just made a big batch of macaroni tuna salad last night! :D)[/li][li]Cool gadgets I read about on the Dope and immediately develop gadget lust for… (tremorviolet)[/li][li]T shirts with goofy sayings on them (Corii) (And then I hardly ever wear them!)[/ul][/li]And will add a few of my own:[ul][li]Crystal figurines, specifically by Swarovski; my curio cabinet of crystals is one of the few things in my house that make it obvious that a girl lives there. ;)[/li][li]Babies and toddlers (maybe because I might not ever have any of my own?).[/li][li]Beer & conversation: I’m a beer fan, but not to where I buy it in the store or keep any at home or anything. However, give me a place with a decent beer selection and someone interesting to talk to (who is also willing to hang out and drink), and I’ll drink way too much and stay out way too late. Every. Single. Time.[/li][li]Baked goods. Cookies, brownies, doughnuts, etc. As evidenced by my standard answer to the question “Are you in shape?” “Yes. Round is a shape.”[/li][li]Legal/political/psychological thrillers (books).[/li][*]Computer wallpaper sites.[/ul]
Oh, fresh-baked bread, especially focaccia-type stuff, bagels, anything like that.
Cooking impliments (knives especially)
New restaurants.
Weird Clothes.
Obscure books or DVDs.
Frank Herbert novels, especially early copies or signed copies of Dune.
Nerd alert!
Books. We’re going to get another bookcase soon. I love books.
Weird foreign food, especially shelf-stable food. I can buy useless things I don’t know how to use and still feel all Domestic Goddess because I’m providing for my family, dammit!! Actually, any kind of food. Like Nigella Lawson, I am not safe in food stores.
I’ll second the cat toys, and add cat furniture. Towers, pedestals, scratching posts, beds, window perches, you name it. Our house is a regular cat Disneyland.
Ooooh…me too! I love all things Hello Kitty. She is my favorite little round-headed creature.
Books. We are running out of space, since my husband has this same addiction, and it seems like heresy to put a book in the garbage.
Cats.
Fresh-baked bread, especially when I’ve made it myself.
Books. A happy day is when the Amazon delivery arrives. Which reminds me, it’s only a month til my beach vacation; I must order some beach books.
Kittens.
And books.
Silver jewelry (Dad calls me and Mom ‘tame crows’ because we’re always looking at the stuff)
Junk food, especially pocky.
Manga.
Ghost in the Shell (mainly the DVDs, but if iI saw other merchandise I’d probably lust for that as well)
General anime, to a lesser extent than the above answer.
Books.
Video games.
Cats, especially kittens. Especially Russian Blue, Singapura, and Abyssinian kittens.
Shoes–boots, gothy shoes, athletic shoes.
Bags–not purse type bags (I hate purses) but backpacks, messenger bags, and other tomboy bags like that. Even though I rarely carry one. I just like bags.
Jackets–especially leather or gothy ones.
Anything having to do with the RPG Shadowrun.
Hellllllooooooo Kitty…can’t resist…so pretty…so girly…
I just realized three more after I sent my last list:
Pizza, as long as it isn’t too weird. I am physically incapable of turning it down if somebody offers me some.
M&M’s.
Chococat. Was never much into Hello Kitty, but I love Chococat.
Camisoles and tank-tops with shelf bras. Because why the hell do I want to mess around with finding a bra when I get dressed? Huh?
T-shirts and other junky crap relating to my obscure media obsessions (Invader Zim, ATHF, Venture Bros., and the like.)
Any ginger-flavored food- or beverage-type product. Especially if I haven’t ever tried it before.
Science fiction books with garish 60’s and 70’s covers. Actually, it doesn’t have to be sci-fi. I just love paperbacks with tacky covers, the more orange and acid green, the better.
Cheap board games at garage sales, discount stores, the Friends of the Library sale. If it shows even the slightest hint of having any strategic depth, and it’s less than $5, it’s MINE. MINE MINE MINE. Because, I ask you, is anyone else at the sale going to appreciate it? No, I didn’t think so.