Word.
(Probably a hijack) I would put myself in this category as well. I recently got called back for a 2nd interview and the interviewer (again) said how impressed he was with me and the fact that I scored a 100% on their computer test. I didn’t get the job. 
Then I later received an email from a national Giraffe-sponsored toy store that I applied to for seasonal help. They turned me down. Guess my talents and experience aren’t giraffe-y enough.
(end hijack)
Sleep. I don’t know of anything that feels better than waking up after a good period of sleep, whether it be a full night or just a good nap where you don’t wake up too late and can still get stuff done that evening.
Internet in general. Despite all the useless crap and spam out there, it is a really good information resource. I don’t take everything verbatim, but I can at least use info off the net to be able to ask semi-informed questions to people who know more about the topic than I do. I’ve been quoted as saying “I don’t know what I did before I had access to the internet”. (Though that’s a lie; I spent a lot of time in the library.)
My DVR. I don’t remember the last time I watched a program (other than live news) that I hadn’t pre-recorded. Now I don’t have to waste time with commercials (and the accompanying volume differential blasting at me).
Health. I have developed several problems over the last five years or so that make me miss my excellent state of health, which I now realize I had taken for granted. Smoking, drinking, poor lifestyles - I never gave it a second thought and took it for granted. Now I wish I could undo the damage faster than I have done it to myself. Good health is vastly underrated.
The unique culture and sublime magic of an evening spent simply passing the time in New Orleans.
The sheer musical brilliance of a really good Grateful Dead show.
The simple beauty of sitting on a beach somewhere and just watching the waves roll in, while enjoying a cold, cheap beer…
Mail delivery again, and not just because this thread made me think of it. This has been a long-held opinion of mine.
Also, modern dentistry. I don’t think most people give much thought to how awesome it is compared to the way it was. Think about having a really sore tooth, and the only thing to do about it is drink a bunch of whiskey and have a guy grab the tooth with pliers, put his boot on your face for leverage, and tug until he yanks it out. We should all pretty much be offering sexual favors to our dentists after every appointment.
RC Cola, Everyday Glass Cleaner, generic chips-a-hoy, chicken hearts, Kumho tires, Rain-X, wasabi and old-school pinball.
Hot showers
Sanitation (in all forms but especially toilets)
A made bed at night
Massages
The Internet (impossible to overrate it)
Independent travel (eg, car, bike, etc)
Fresh Milk
If you think I’m mad about that last one, go live in France for a bit. No fresh milk or proper cream cheese ANYWHERE!
Tap water.
The water utility in general. For $200/year we get something like 800 showers, 365 days of clean dishes, clean faces and clean teeth, about 4000 toilet flushes, about 500 gallons of drinking water, healthy plants and lots of dunks in the baby pool (for the dog).
Electricity for me costs about $200 every 2 months, and heat about $200 every 3 months. Water is a steal!
You think yours is a steal; I get mine out of the ground: for free!
(Well, the cost of an electric pump anyway.)
TV.
There are things on TV nowadays that are far better than 90% of what we see in movies. Things like The Wire, Friday Night Lights, The Sopranos, Rescue Me and many others.
Yeah but think of how far my water has to travel and how many people, machines and chemicals it takes to deliver it cleanly to my house. Water from the ground + a pump is not as amazing to me! 
Yes, it amazes me that I can get cool clean drinking water from a hole in my yard, for free.
All that fresh water underground, I’d like to go down there. Is it just whatever seeps into the well from the immediate surroundings or are there little creeks, rivers, huge underground lakes?
Either way it’s amazing.
The absolute most underrated thing is being overrated. As a corollary, the most overrated thing is being underrated. Everyone always talks about the most underrated athlete or the most underrated band or movie or product or whatever. Screw that, being underrated sucks because it means you have qualities that people don’t think are being recongized. You know what? I’d rather be overrated. An overrated athlete gets paid a lot more than an underrated one. An overrated product sells more than it probably should. An overrated movie or band gets exposure and respect and fans.
Water, air, electricity, warmth, plumbing, health, the sun, etc, are not underrated.
If you asked someone to rate those things, they would usually rate very highly. We just don’t go around rating things all day.
I’ll second libraries, and the quality of tv as compared to movies.
The availability of pet-friendly hotels in the USA is vastly under-rated.
Flying.
If you actually sit up front and watch the air traffic control system work, it’s kind of humbling.
The entire array of ground controllers, tower controllers, approach/departure controllers, center controllers, and their radars, vortacs, ndbs, localizers, glideslopes, and satellites all working in concert to move thousands of planes simultaneously through the air is really mind-boggling. Sometimes it’s overwhelming to think about the training and design that went into all this, and their ability to coordinate airplanes whose speeds can range from 90 mph to 900 mph, along with the computer systems which can reserve the appropriate blocks of airspace for each airplane’s planned movement, watch and predict weather, etc. etc. This stuff runs 24/7 worldwide, and seldom hiccups.
The fact that a person can buy a relatively inexpensive ticket, sit in a comfortable seat for a few hours, and have this enormous system work to place him quickly on the other side of the world, is really underrated.
Here is a pictorial of a day’s air traffic in the US.
I agree with most of what’s already been said, and add spreadsheet programs. As someone who started accounting with a 10-key, green ledger paper, and various colored pencils, my livelihood was made extraordinarily easier when I first got Lotus 1-2-3 back in the dawn of time.
Also, being able to buy cheap bananas in Iowa is great.
Agreed. It works very well as dark and violent comedy, and it was meant to.
I do! Hell, an iphone even looks like the future. It boggles my mind that I can access something very close to the sum total of human knowledge at any time, anywhere I might actually go.
The absence of pain. I’m not in pain right now, but everytime I’ve been in unremitting, chronic pain, having it go away is sublime.
Quiet. Oh, God - quiet. My mom is coming to visit next week and she’s a TV addict. Literally cannot be in a room without one. I already can’t wait for her to go home.
Eating healthfully. I do this way too infrequently. Everytime I eat healthfully for a few days or more, I feel wonderful - more energy, feel stronger, sleep better and can function on less sleep.
A clean house. It’s even better if you combine it with quiet. Ahhhh.