I’m not sure if this qualifies, but it was too damned funny not to throw in here:
A transcript of a typical (FILL IN YOUR FAVORITE SDMB FORUM) exchange. With cigarettes out of fashion for over 3000 years, they will surely come to the conclusion that Satan was combustible. God knows what they would draw from some of the more esoteric sigs. They’ll research me, find out I was a Kentuckian, and probably come to the wrong conclusion about mine, though.
Sir
“1500” years.
Pants.
In the year 4000 an archaeologist will discover the remains of a Gap store (or Abercrombie & Fitch or wherever teens are shopping these days) and determine from the abundance of huge, baggy pants that Americans circa 2000 were a grossly obese lot. God forbid the poor bastard should unearth a cache of used pants and try to glean physiological information from the wear patterns. Strangely, it appears that humans of the era had extremely short femurs, very long tibias, and sat on their lower backs.
Condoms.
Cellular phones.
Typewriters and computer keyboards.
Keys.
This reminds me of a Calvin & Hobbes cartoon where Calvin digs up a bunch of junk from his backyard and thinks it’s a dinosaur. He finds a glass bottle, some silverware, puts it together and calls it the Calvinosaurus (I believe, though it’s been a while).
This really makes sense to me. We’ve moved onto plastic bottles for most soft drinks, how far away is something beyond that, or something beyond eating utensils (maybe food just floating up to our mouths :)).
By 4000 A.D., I can imagine pretty much anything we use regularly today seeming strange to them.
Definitely SPOOFE. I’m currently saving up for one.
My friend composed the music for my final film project last semester. I thought we could just tape it, and he suggested using a mini-disc. It was cheap and it sounded great!
Reuseable, portable, great sound, and you can record on the portable systems (don’t need to buy a separate burner)…sign me up! 
Nothing to add really to the OP, but I’ll continue the hijack by saying “me too” to the Minidisc angle. I’ve used it for recording live music (of less than legal origins) since I got it last summer at several gigs.
I mainly got it for recording my own original music. It was great to record CD-quality demos at band practice each week.
Finally, they beat the hell out of portable mp3 players, too.
[/hijack]
Darn it, DDG, I walked around my house all night last night after reading your post and evaluating objects, trying to imagine if their use would be obvious.
Sports Merchandise
“Clearly these Americans had a strict clan system, with families worshipping various icons, such as buffaloes, cardinals, and sharks.” says Synthar-Three. “They appear on garments, drinking vessels, and small fabric relics, shaped like animals and filled with small beads. The beads must have once formed a more solid stuffing, because the relics flop around in a most disarming way. Although, it’s sort of cute … maybe we could sell reproductions in our 20th century gift shop.”
“But what, exactly, did the Jets clan worship?” asks RamRom-Two, in a puzzled voice.
“No one knows, RamRom-Two … that’s a mystery.”
“If your theory is correct, Synthar-Three, why would anyone worship a man with a giant white ball for a head?”
“Shut up, RamRom-Two.”
I wonder if anything from our civilization we still be standing 4000 years from now. The pyramids are that old but what works of the 19th or 20th century will endure for that time, if anything?
The Statue of Liberty will, albeit from the waist up. “Give your tired, your poor, your knuckle-walking silverbacks yearning to breathe free.”
Maybe the NORAD complex inside the mountain will remain intact.
Maybe the Empire State Building.
Out in the desert, most of Las Vegas will probably still be standing. (Heaven forbid how Synthar-Three will interpet that The Strip!)
Plenty of mothballed-bomber depots in the desert will, too.
Maybe the repository at Fort Knox.
A whole bunch of geostationary satellites still in orbit.
And six Apollo lunar module first-stages left on the Moon.
Oh, and Dick Clark. Definitely Dick Clark.
What would really be interesting is if they actually figured those monstrosities out.
“Wow, they really must have hated their women back then! I’m glad we’re living in a much more enlightened time!”
The 20oz plastic bottle.
Perhaps the hygiene level in the 20th century was so low that very few people kept their teeth beyond adolesence, forcing them to exist on a liquid diet.
The Yugo;
Archaeologists in the distant future will uncover a Yugo, and presume it was some sort of primitive can-opener.
Cow tools.
Aside from buildings and cars and various plastic objects, I try to think of things that would baffle scientists…
I think religious objects and things of that nature will be completely understood… after all, the big 3 have been around for a while, no reason to think they would disappear.
things to baffle…
personalized things like t-shirts with loved ones pictures on them.
many many toys.
what I think that sociologists of the year 4000 would be most baffled by (I really like the word baffle) would be things that we do, and why we do them… what would they make of the Napster controversy, and Monica-gate, and things like PETA and NORML…
I was going to say “Woopie” cushions, handshake buzzers and fake vomit, but knowing mankind those will probably still be around.
Anything and everything from the Sharper Image catalogue will probably keep them guessing for centuries. Most of the items have me guessing right now. Why would anyone buy that stuff?
The plastic Flinstones toys in Happy Meal bags oughta’ throw 'em a loop.
“Looks like it’s from the Stone Age, but it’s made of plastic.”
(Of course if Science is still on the same track it is now, they’ll date them 2 MILLION years old)
“PETA, NORML? These are obviously acronyms, RamRom-Two, but I can’t figure out for the life of me what the devil they mean.”
“Those primitives must not have had a very good grasp of the use of acronyms. Now now, Synthar-Three, gather your belongings or we’ll be late for the NAMBLA meeting.”
Mount Rushmore.
Thousands of CDROMS, but these will only baffle until someone digs up the appropriate reading apparatus.
(Which will be found in a bunker in Montana.)
It’s hard to imagine our entire civilization disappearing, though. We’re forever preserved on digital media. Will that last forever?
Probably every Far Side. And maybe lots of other comics, too.