Baby monitors. Here is another one of those things where used some bare bones ones and some fancy-schmacy ones, but the best one we've found is somewhere in the middle. Off the top of my head between our house, both sets of grandparents' houses and various hotels, we've used at least 7 different baby monitors that range in price from $19.99 to $250+. The Safety 1st High Definition Digital Monitor (which costs $69) is my favorite by far.
What I love about this monitor is the simplicity. It is a clean and simple in every way. It runs on one clear channel and we have never had an interference issue with it (other monitors, Blackberries, clock radios, cordless phones, wireless networks - nothing!). The nursery console is small (about 2.5" square) and has a great microphone. The battery life is also fantastic. We can often unplug the parent unit during the morning nap and run around until mid-afternoon without having it lose its charge. The belt clip doubles as the stand to keep it upright on your night table. The beauty of the design is that it is so small and so simple that it virtually blends into the background when you're not using it.
Yes, it doesn't have fancy features like temperature, music, light shows, video or intercom, but do you really need them? What you need in a baby monitor is a clear sound of what's going on in the tot(s)'s room without snap-crackle-pop of interference or being tethered to the power cable (because of poor battery life). This one is a winner.