Ok, so I sheepishly add my project here even though it's really not terribly creative considering the other amazing entries here. Thought I'd add it anyway as a source of inspiration maybe?
6 foot length of copper plumbing - $3.00
2 hooks - $0.50 each = $1.00
end caps - $0.30 each= $0.60
I'd been brainstorming about a nice curtain rod for my kitchen for a while and happened upon a cool looking aged, coppery one in a boutique store once and it was actually a length of copper plumbing!
I borrowed my bro's tube cutter, screwed the hooks into the wall, capped the length of plumbing and hung it up! Now I just need to make a nice curtain! And since I liked that so much I made the same rods for every window in our apt.
note the beautiful orange glint it gives off. Fortunately I declined to polish the rods so they look nice and aged. Please excuse the leftover party decorations!