Grobag. I'm sure you've seen similar styles at Babies R Us or other baby stores. There are similar style from Halo Innovations or the Slumbersac. We got a few of the halo from our baby shower that we used until she was about 6 months old.
I like the Grobag more because of the way it's made. It seems like it's easier to put a sleeping baby in the Grobag because it flips open completely and you only have to slide in one arm instead of two. The one we have has snaps on the left shoulder and zippers on the side. It's also thicker than the halo bags. They do come in different thickness. The thickness for our grow bag is 2.5 TOG. The best way to explain that is our house can be around 63 degrees and she stays warm and toasty with just a night shirt on underneath.
Since it open up completely I've been able to change her diaper while she's asleep without waking her up. That's a huge plus in my book. The Grobag company has a few different product from swaddlers and Grobags to fitted blankets for bigger kids. My daughter loves her Grobag, now she's old enough to understand when I lay it out she sits on it waiting to be zipped up for "night night time". This are especially great for cooler climates. I hope this helps.