Infant gowns--the BEST for newborns--SO comfy, so easy, and great for the million diaper changes. After they start moving around a little more, I like to switch to footie jammies with zippers.
Changing pad/cushion--I haven't seen this one mentioned yet, but we used it constantly when DD was tiny. This is the foam thing with the slightly raised sides that you buy to put on your changing table, and we bought it to put on DD's dresser to use for diaper changes. But we ended up carrying it all around the house for the first 2 months--it was great for her to take naps on outside, on the couch, living room floor, etc. When she was really tiny and slept a lot, I even put it in our clawfoot tub so she could nap there while I took a shower. I liked that it was soft and comfortable for her, kept her and the house clean (no dog hair on the baby, no baby poop blowouts on the couch/rug), safe (it has a fuzzy cover that fits on like a fitted sheet, so it was cozy without being a suffocation hazard like blankets could be), and it kept her from rolling off the edge of the couch, or into the cracks of the couch cushion (where the back meets the seat). She and I took a lot of naps together on the couch like that when she was tiny.
baby bouncer and/or swing -- my DD (and I) LOVED her full sized swing when she got to be about 6 weeks to 6 months old--she could nap in there for like FOUR hours, where she would only nap for an hour or less if she was on her back, and sometimes it was the only thing that could help her fall asleep on bad nights. That big swing was worth every square inch it took up, and more. Bouncers or travel swings are good to help you make dinner or catch a quick shower--but the little swings don't have the power of a big swing, and I wouldn't give my big swing up for anything.
Carriers--I used a sling a lot with DD, but honestly didn't like it much when she was tiny--I had a very difficult time getting her positioned so her head wouldn't collapse on her chest. This time I got a Sleepy Wrap, and I'm hoping it will be better.
Kiddopotamus velcro swaddler wraps--swaddling was essential for DD's nighttime sleep for many months. We tried regular swaddle blankets, and different brands of velcro wraps, but she could escape all of them and she'd wake herself up and be unhappy. With this brand (that our local CVS/RiteAid actually carries), she was snug and secure and slept great!
Bassinet -- she slept in hers, right next to our bed, for the first 6 months. We thought we'd try cosleeping, but it just did not work for us--we all slept better when she had her own space. BUT, I wanted to have her right next to me at first so I could check on her, and so all the nighttime feeds/changes would be easier. It was hard (on me, not her so much!) to move her down the hall to her room/crib at 6 mos, but she had outgrown the bassinet, and eventually it did feel good for DH and I to get some bedroom privacy back.
If you're BFing and going back to work, a good Medela breast pump
The Happiest Baby on the Block book
Breast pads and nursing bras/tanks
Baby monitor with video screen -- we didn't use this a lot when she was tiny, but as soon as she started napping and sleeping in her own room more, this became invaluable. She can be a light sleeper (not all the time, but sometimes), and we have old creaky wood stairs and floors, so I can't go check on her without risking waking her up. Also, sometimes she makes noise in her sleep or will wake up a little bit--if I go check on her then, it is guaranteed to totally wake her up. If I just look at her on the monitor, I can see if she is falling back asleep on her own and I should leave her alone, or if she is actually awake/upset, and she needs me to come to her.
Infant bucket car seat--perfect for removing a sleeping baby from the car without waking her, and also makes a handy travel napping spot so your baby can catch a quick, comfy nap at a restaurant, friend's house, etc. if the need arises. Also good for keeping baby warm during winter travel without having to bundle her up in a snowsuit--just pop her in the warm seat in the house, cover her in a cozy blanket, put a hat on her, and she's good to go.