I started thrift store shopping many years ago when I was young and poor. Many of my son's clothing items, books and toys have come from there since he was little. As he got into school, I was able to find some of his school uniform clothing there as well (polo shirts in certain colors and khaki/navy pants).
Now my son is 18 and will go look at the thrift store first when there is something he wants. One trip last fall he found a perfect-condition full length black wool coat, for $7 (savings of hundreds...) I found a nice watch for me for $3 (just needed batteries) on the same trip. He was in drama, so it was a big savings when looking for stuff for that as well (the school could provide some, but not all of the clothing, especially since he's a big kid, somewhere around 6' 3"?)
Some of my favorite finds:
A rocking chair 90% off, so $7.50
Cast iron skillets-- one Wagner's, one similar quality, $2 total (although I did use a $10 "loyalty" card to bring the price down, they were on sale at $12
The wool coat mentioned above, perfect for my son, and even though he's the one who found it, I'm claiming it...
Enameled deep cast iron skillet with lid, less than $5
Fabric-- small bag stuffed with hand died batik fabric, $2-- I'll use a lot of it for quilting, but might make a tote bag with some of it.
But really, over the last 18 years I've found so many good items, from clothing to furniture.