What a useful word!
I’m usually pretty good about throwing things away if I don’t need them, with one exception. Any craft item gets thrown into a box with the idea that “one day” I will sort through the stuff and use it. :rolleyes: This includes sequins I pull off of sequin fabric, 1/4 strips of beaded fringe, 2 inch strips of sequin trim, unusual beads that I only have one of, etc.
So what kind of mathom do you have lurking in your house?
Oh I keep lots of stuff. Too much in fact. I keep receipts and little pieces of paper. I force myself to clean my purse out every so often so I can get rid of stuff that I don’t need anymore.
I keep clothes that don’t fit me and probably will never fit me again. I also keep gifts (clothes) that relatives gave me but I hated. Occasionally, I’ll shift them around in the closet so it looks like they’ve been worn.
I keep cd cases. I always manage to lose the cd but I’m never short of any cases for them. 
Mostly my writings. Even my bad stuff from when I was in 8th grade. My journals. My letters to friends. Very rarely will I throw anything like that away, thinking that “someday” I’m going to sit down and read through it all and see the journey I’ve taken from the girl that I was to the woman that I am.
That day hasn’t happened yet. 
“Mathom” is unquestionably my favorite word from the LotR. I find it very useful, because I’m the ultimate packrat–were I immersed in hobbit society (where I clearly belong), I could go to a birthday party every day of the year and not clear my home of mathoms.
I have laser rifles, Tesla coils, gyroscopes, notebooks, crates and bags and piles of “craft” supplies, a spare sewing machine, several utterly worthless computers of various types, at least one musical instrument that is physically impossible for me to play (it would take two sets of hands to reach all the finger holes, although there are only about six). I have knicknacks, bells, and odd-looking stones. And yes, I do believe <rummaging about in a closet> a brown paper package all tied up with string! I wonder what could be in it?