Woodpeckers. I love woodpeckers - probably because they don't try to build homes on my home (or at least they haven't yet). When I first began birding figuring out whether the woodpecker common to my feeder was a downy or a hairy was one of the most frustrating identifications I had to work through. I didn't actually even know there were woodpeckers other than the Pileated who I knew because of a cartoon show that came on when I was a kid, Woody Woodpecker. When these guys showed up in my awareness not only was I blown away that there were more species of woodpecker than I could have ever imagined, but that two of the many species looked almost identical.
Downy and Hairy Woodpeckers are relatively common birds to see in your neighborhood. Here is a great article from Birds and Bloom with some simple tools for telling these two apart.