Wilton makes a couple of sets of cookie cutters. I have the animal ones and they've been pretty useful to me. Other than that, icing colors, round tips, disposable bags, etc.
Hobby Lobby sells their own treat bags, which fit cookies nicely and don't cost as much as the Wilton ones. They're in the party stuff aisle.
I like my Doboard, cookie sheets from Sam's, parchment paper, these 2 oz. squeeze bottles for Glace' icing, http://www.countrykitchensa.com/catalog/product.aspx?T=1&productId=619471, Americolor food color gel and lots and lots of cool cookie cutters!
I agree the Wilton animal set has a bunch of cute cutters for the price.
I got the Wilton 101 piece cookie cutters from WalMart for like $8, I have relly enjoyed having them, lots of fun shapes.
For details on sugar cookies, I have those tiny decorating tips (I dont remember what their numbers are).
I got my cookie sheets at Sam's, I like them a lot.
I got a pastry scraper (that I also use to help smooth over frosting on round cakes.
Food coloring.
Decorating bags.
Paint Brushes, be careful what kind you use though, some of them shd bristles really easily (instead of outlining the cookies with a piping bag and then flooding the middle, I paint on the outline of the cookie in thinned out royal icing and then paint in the middle, the same way you would flood a cookie, but I get a slightly thinner layer which I like better)
I make more than just sugar cookies too, and I have a ton of recipes! I keep them in their own separate file on my computer so I can find them easily (they were kind of all over the place lol, all the cake and cookies recipes in the same massive file...it sucked lol).
I'm not a cookie guru but I wanted to second the motion for the dobord--it seems like a so what, but it's thee bombshafreakingbomb.
I love and use all my graduated cookie cutter sets. Love them.
Silpats or a Roulpat and a good rolling pin!
I like the Wilton cooling racks, I always use my coupon on those. I swear I never have enough.
Good cookie sheets
I'm asking a stupid question but what's a dobord? I haven't gotten into the cookie making yet but that's the next step.
A Dobord is a 16"(?) square wood surface that has adjustable side rails. The side rails can be unscrewed and set to sit above the surface in different heights, sliding along metal grooves, and you roll a large rolling pin along the surface of the rails. The rolling pin comes with the Dobord. Sorry if my explanation is confusing! I bought one for about $72, which included shipping. It's a fantastic tool!
the things I can not do with out when doing cookies:
1st: my "rolling" board-i made this with a large piece of masonite board and put "edges" on it with square dowels. that way my dough gets rolled out evenly and the same with everybatch.
2nd: parchement paper-no waiting for cookies to cool in order to use that cookie sheet...just put a sheet of it on the board before rolling
3rd: NFSC recipe, found in the recipe section. does not puff up and spread like other recipes and it tastes great
1. Uncomplicated cookie cutters. You can add all the detail you want, or keep it simple. I prefer metal cutters, as the plastic ones always seem to leave a little ragged edge (which is easy to smooth, just another step).
2. My Dobord! I have rolling pin rings that I loved before hearing about the dobord, and getting a surprise for Valentine's Day. I know others have rigged their own boards with great success.
3. Small laminator- if I see something I don't have a cutter for (and am only doing a few) I resize the picture to what I want, then print. If doing more than a couple or I think I may do that shape again sometime, it is easy to laminate, then use as a template.
4. Multiples of the small round decorating tips. It seems the one I want is always on another bag! I guess assembly line coloring would help with that issue!
5. A supportive family who understand that cookies are more important than food, cleaning, laundry, etc!!!
6. antonia74's cookie tutorial here on cake central!
Parchment sheets. Rolling cookie dough between those two sheets is so quick and easy. No flour, no mess.
A tip, if you use the NFSC recipe, reduce the baking soda by 1/2 and use cake flour instead of regular flour. Your cookies will not spread so much.
Also, keep your cookie sheets in the fridge before baking.
Good cookie sheets can't be beat. I have the set of 2 from Sam's (around $18-$23, I think). LOVE them! Nothing sticks to them and they're great for any kind of cookie - even dog cookies.
Ice cream scoops (some people call them 'dishers') in varying sizes are also a must for me. That way my cookies are always uniform in size and bake evenly. I use everything from size #16 all the way down to #60. I buy them at a restaurant supply store and they're dirt cheap.
The cookie scoops are great! Mine are from Pampered chef, but Walmart even carries a couple of sizes. I have 3- 1T, 2T, 3T. Easy way to get consistent "normal", big (my regular!) and monster cookies. If you get invited to PC parties, they aren't expensive, and certainly something that won't end up tucked away.
I just thought of a couple other things
Those lovely little glass bowls from Pampered Chef--Great for small amounts of different colored icing. They come in a set of 6/lids! Love those!
Glass batter bowls/lids
Also there is a line of Martha Stewart glass containers with lids that I love also for storing royal icing. I love that the lid and bowl have markings that show that they go together!
My bench scraper and Americolors - Super Black gel!
let's see... my "must-haves" for decorated cookies would have to include:
* a great recipe (like the NFSC recipe)
* waxed paper (for rolling) and parchment paper (for baking)
* a large rolling pin and those plastic rods keep what you roll at the same height
* small round plastic containers with tops for mixing colors of icing
* lots of small zip-locking freezer bags (for icing)
* couplers and tips in sizes 00 - 5
* a good thin bladed knife (like an exacto) for cutting my own shapes
* large jelly roll pans with sides at least 1 inch deep (to put cookies on while the icing dries)
* inspiration from the amazingly wonderful cookie makers here on CC!
Let's see, must haves... well, I love my "smart" rolling pin from ck, my sheet pans and Silpats I bought from Martha Stewart back in the days when her company had those things. I have tons of colored sugars and lots of plastic squeeze bottles (like the ones you use for chocolate) for mass flooding of royal icing. I also love my palette knife for anything fondant and I'm addicted to copper cookie cutters. They're the only kind I use.
But most importantly, HAVE FUN!! 
stacking cooling racks. they save a TON of space.
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