Good quality Baby Monitors. We have two of these: One super-cheap and mostly annoying (hard to tell if it's on or getting signal, no battery, etc). One nice sony with built-in rechargeable battery, signal monitor, status-led, etc. The rechargeable is great as Dad can do things like take out the garbage and still listen in on baby.
Crib Bumper -We didn't get one of these in the the second time around and, on several occasions found one of our twins with an arm or leg stuck in his crib slats. Not really stuck mind you, but for a moment. We like the very basic and inexpensive "breathable crib bumper". It basically keeps Baby and their accessories (pacifiers, small stuffed animals, etc) in the crib and early-on prevents bonking against the side of the crib.