I'd say books, but a close second is simply cheap stuff!
I keep an eye on a 'deals' site in the UK, and I'm often treating myself to bits n pieces I normally wouldn't buy.
Finally moved into the 21st century when I got a refurbished android phone for £20

Finally replaced the old call n text Nokia.
Also organising my flat with a set of 20 box files I bought for about £11, kinda funny to see the UPS man hefting 4 big boxes up the stairs, knowing how little I paid, I can't see how they profit
As for books, finally updated my Stephen King collection, 26 nice, shiny new hardbacks for the same in pounds, again, kinda felt bad for the courier, dragging them up 3 flights of stairs to me!
Now got a lovely orderly bookcase that just goes KingKingKingKing... for a shelf and a half
Oh and having interest in 'The Hunger Games' movie trailer, I looked around for the book, but couldn't find it new for less than seven quid, then found somewhere selling the paperback trilogy for a fiver! Bargain.