Bricks and boards are quick and cheap.
Have you been in my living room, Naj?
Yup. 8 12x12x4 garden blocks and 4 12x4x1 plywood board (they'll cut them for you at Home Depot - just buy a slab and tell them the measurements). One can of paint and you have an instant bookshelf. I'd be lost without mine and refuse to part with it.
Or milk crates... I stole a lot of milk crates in college, I still have enough to line two walls floor to ceiling with very functional shelves :~D