Room dividers tend to fall into two categories. The first is ugly and inefficient and the second is overpriced and uber modern. Forget Craigslist, do away with retail, all you need to do is head to your local hardware store for a few basic supplies.
1. Rope: We've seen walls made of rope and they're not only attractive, but efficient. They break up the space without making either area a dark cave void of light. Try making a full square frame of wood that is custom fit to your space. It should allow you to not attach it to walls or floors, perfect for renters.
2. Thin Plywood: One of my favorite projects to date were these hanging wooden walls. They were easy to make and inexpensive. Even if you're not hanging it from the ceiling, it could easily be adapted for your own personal needs with feet or wheels.

3. T Buckles & Wire: Also called Turnbuckles. Sure you can pick up a kit at IKEA to hang a set of curtains, but they're kind of, well, in short, sucky. Check out the hardware aisle and look for T Buckles to do your dirty work. They'll hold true and tackle any weight fabric. Check out this project where we hung 30 feet of curtains for $40.
4. Drop Cloth: Drop cloth is pretty darn cheap and already seamed for you when purchased at the hardware store. You can also look at other plastic alternatives, but the cloth can easily be ironed and give you a good look quick. Stretch it, hang it, paint it or dye it. The options are endless.
5. Fencing Supplies: Try making low railings, industrial looking dividers out of chain link and more. Different stores will carry different pieces. Don't forget to check out the corrugated roofing panels, they're cheap and can be easily lit with rope lights!
(Images: Sarah Rae)

Commercial Flour Sa...
Don't forget the currently ubiquitous pallet.
http://myfriendstaci.com/2010/09/07/wood-pallet-room-divider/
I use drop cloth on my closet and it looks perfect.
I hate to sound pedantic, but I think the writer meant to say that drop cloths are hemmed before you buy them, not seamed.
Hems turn under raw edges and cover them. Seams join two pieces of material.
And while some larger drop cloths might indeed be seamed, many are not.
I used clear corrugated plastic panels hung from the ceiling by "s" hooks and eye screws. They provide separation and privacy but still allow the light to penetrate.
Floor to ceiling closet built out of mdf and scaffolding. The side of the closet swings open to function as a door: http://ploefff.files.wordpress.com/2011/10/ziebell5.jpg
Very cool!
And of course there's always the bookcase divider. A tried and true method throughout the decades.
I saw a really cool art installation in Miami where the artist had taken silk flowers and stitched them together at the ends of the petals...he made a huge colorful lacelike curtain out of it and I thought it would make a great room divider in a space where privacy was not the issue.