I've wanted to make a rug for my home office for a while. The previous owner did the floor in stone and used the room as a dining room. That doesn't work for me, because the office is the only place in the apartment that can house bookshelves. The stone floor is still nice, but it can be uncomfortable when I want to sit on the floor and look at the knitting books on the bottom bookshelves.
I found a free pattern for a doily rag rug, and used a purpose-bought king-size sheet set torn into 2.5cm/1" strips for the yarn, crocheting loosely with a 10mm hook. Because I was using rags and not the strands of worsted yarn called for in the pattern, mine came out bigger, at 109cm/43" — which turned out to be the perfect size for the amount of available open floor space.
I had to improvise the last few rounds, simply because I was running out of yarn (see the leftover ball in the photo above). One thing I noticed about crocheting with rag yarn instead of the usual spun yarn is that the stitches come out a lot wider — a single stitch can easily fill a ch 2 or even ch 3 space.
One nice bonus is that the rug doesn't seem to slip around a lot on the floor, despite not having any non-slip backing. I think this might be from the texture of the stitches themselves. The other bonus is that since it's made from 100% cotton sheets, it's completely machine washable and dryable.
Edit: A lot of the patterns on-line mention this, and I'm going to mention it here — working at this gauge, on something this big, is very hard on one's hands and arms. I finished the rug in four days, not working more than a couple of rounds at a time before doing something else, and I still wound up with very inflamed arm muscles. Today is the third day after finishing the rug, and the first day I'm not in pain (although I still was a bit this morning). Take your time if you make one of these — finishing in a week is still plenty fast.