When he was a first year student at the MIT Media Labs, Paulo Blikstein built a water-based device that could handle two Boolean operations, AND and XOR. And after making a few hardware revisions, ended up building a 4-bit adder out of some strategically cut acrylic and rubber tubing.
"The idea of the project was to build a device that could do computation without electrons (well, not considering the electrons in water itself). Water was a interesting choice, in fact, Fluidics is a very important field of study that is widely used in aerospace or mission-critical applications, where electronic control devices don't offer the needed reliability or cannot support the environment. Also, Fluidics has been use in military equipment in order to prevent malfunction in a nuclear war, when electric devices cease to work."
It's not going to revolutionize desktop computer, but interesting weekend reading nonetheless.