I enjoy following innovative designs of familiar things. My google reader is filled with items involving vases made from soda bottles and see-through refrigerators.
The most recent thing I've found in my internet 'trolling that would be most useful in a classroom is the pencil printer. Useful because:
a) printer ink is more expensive than blood.
b) in 98% of classrooms, teachers pay for printer ink out of their own pocket.
c) a classroom does a boatload of printing.
d) the printers themselves are loud, inefficient, and on the verge of crumbling at the very thought of being housed in the same room with 35 other bodies.
e) classrooms are filled to the brim with pencil stubs. I found one in a light fixture once. Another time, I found a whole stash behind a bunch of books in the shelves - as if some kid was a squirrel and pencil stubs were his acorns.
f) half the things I print out for teaching become obsolete within a year, so I might as well erase the old info in order to reprint new info on the same paper.
This pencil stub printer probably costs thousands of dollars though. So in the end, a regular printer is still more immediately affordable.