The "big three" U.S. "legacy" airlines (American, Delta, United) are some of the largest operators of 757s today. Delta (shown here) maintains the world's largest fleet. Southwest, jetBlue, and Alaska Airlines operate exactly zero of them.
This originally started out as a "one more thing" on my recent discussion of getting to St. Thomas, U.S. Virgin Islands, from the Lower 48 states. It quickly became apparent that my gushing (in that post) over the venerable Boeing 757 aircraft was going to jump the shark, and a new story was born!
You've most likely flown on this plane, and might readily recognize it as seeming to go on for days (its dimensions -- long like a larger plane, narrow like a smaller one -- create the illusion of extreme overall length) and seeming to be rather high off the ground when parked. You might also recognize that the main boarding door is positioned about a third of the way from the plane's nose, just in front of the wing, allowing passengers to turn left or right when they board, which we feel cuts down on some congestion in the aisle.
We love this girl. She entered service in the early 1980s as a replacement to the (also venerable) Boeing 727, and with over 1,000 copies sold until production ceased in 2004 has been a mainstay of the "middle of market" routes ever since. They're special because they can fly further and carry more passengers than the Boeing 737 (the most popular commercial aircraft in history, and the only thing flown by Southwest) or the Airbus A320 series, but are smaller (and thus cheaper to operate) than the big twin-aisle high capacity jets that fly the international routes between big cities. This makes them perfect for longer trips on less popular routes (enabling, for example, Iceland Air to have served such routes for years) and equally perfect for shorter trips on more popular routes. For example, in early March 2016 United Airlines became the operator of the largest regularly scheduled flight out of Washington Reagan National Airport (DCA) when it replaced the 737 with a 757-300 on its once daily and always full route to Denver.
The 757 also shows up in novel, non-commercial settings. The U.S. Air Force uses a version of it, "Air Force Two", as the usual air transportation for the Vice President (or for the President when landing in an airport too small for the much larger Boeing 747-based "Air Force One"). Several nations use them as military transport. And, yes, even the made-famous-by-the-2016-election "TRUMP" plane is a modified Boeing 757, making it one of the largest privately owned planes in the world.
All this said, these planes are showing their age. The one I am sitting in as I write this is 1990s incarnate, complete with the TV screens that drop down from the ceiling in the aisle when showing a movie. Boeing seems to have felt that there wasn't to be a market for a 757 replacement, instead hoping that the latest version of the 737 could stand in. But even with the range improvements of the 737 MAX, it just can't fully replace the 757's sweet spot blend of range and size. Complicating this is the new Airbus A321 NEO, with which greater range seems like it will outsell the new 737, but still can't match the 757 on total number of seats.
Geeks like us are hoping that Boeing might prioritize development of a 757 replacement in the medium-term (think 2020s). It certainly looks like an economic imperative if they're to avoid losing business to European rival Airbus. Until then, we'll have to geek out on the nostalgia when we show up at our gates and realize "Yes! We're flying a 757 today!"