In 50 years or so, it will be disposable nappies (diapers).
Those things will NEVER decompose, and as for their contents, well…
Judging from the condition of the living room and our sons’ rooms, either Lego/Duplo blocks or Matchbox cars 
Don’t know if this counts as man-made but it does come from quarries: Gravel. If that doesn’t count then I pick ballpoint pens.
I’d say screws (bolts). I think there are more of them than there are nails. There isn’t a single nail in my office that I know of, but probably a couple hundred screws - mostly in computers and electrical appliances, some in steel furniture.
There are a lot of shoes too. I’m sure more people use shoes than they use shovels, buttons or light bulbs.
I second screws, and transistors if those count.
Would stitches count? It’s a noun, but I’m not sure if it’s an object. If it does, I’m guessing there’s more stitches on people and in closets in an average building than nails/screws/rivets.
cigarettes!
Sperm.
Matches. I don’t know how many nails there are in my apartment’s construction, but there are at least 1500 matches.
All good answers. I thought about buttons, and matches too. Stitches is an interesting one. Along the same lines, how about bits of bytes? – electronic data. Kinda esoteric, but I will put it up for consideration.
Lumber? Yeah but every stud in my house has 4 nails. And that doesn’t count the drywall (it’s nailed, not screwed).
Diapers
cans
staples
nails
shingles
McDonalds’ hamburger wrappers.
I was thinking people. There are over 6 billion of them, all man (and woman) made. But then I saw lieu’s answer, and that has to be it. Heck, I bet I’ve made more than 6 billion sperm all by myself. (Just imagine if you add in the one’s I’ve had help with. ;))
Spoons. 
Nearly every culture on the planet uses them in one form or another, even the Asian cultures that use chopsticks rather than the western fork/ knife combo.
shrug That was the first thought that came to mind as I was reading the OP…I have to admit, there probably are more nails…and certainly are more sperm. But, oh well. That’s my answer, and I’m sticking with it.
-Stil
Bricks? They’ve been around for a long time, they’re used by almost all societies for building, and they stand up well to age, so lots of the really old ones should still be around in addition to the newer ones.
Printed words and communicative marks.
Bullets?
Although we control the language and are therefore able to unfairly attribute production of it to a better class of animal, I’d have to go with bullshit…
Bricks are another good idea. That would also include adobe. Lasts a long time.
I also love the AOL disc comment.
Bullets. Yep. How many bullets must you have to stockpile to supply an army? 5,000 for each soldier?
And anyone that owns guns probably has at least 50. Probably many more.
My Dad gave me a case of .22s. I think there are 5000 of them in there (long story, he was worried about Y2K).