From their website:
Ronja is an Open-Hardware optical datalink that connects two PC’s point-to-point. Ronja’s design is licensed under the GNU Public License: you get all the necessary documentations and construction guides free. The construction costs are minimal, it’s probably the cheapest wireless system ever. The operation is very reliable and immune to interference
And here’s the link: http://atrey.karlin.mff.cuni.cz/~clock/twibright/ronja/