Useful machines built with LEGO
Here are some examples that caught my attention in the last years:


Cambridge scientists are using LEGO Mindstorms in their lab instead of a professional robot, saving them large amounts of money.
The Kno tablet development team is using a LEGO rig to perform automated stress tests on their devices.
University of Southhampton is using LEGO bricks as the structural frame for a cheaper supercomputer (warning: they use the word legos).  
Building custom encasing for electronics like this monster three-motherboard PC.
For automatic milling and sculpting, there are also plenty of examples of LEGO CNC machines. They can be used to prototype parts, make molds, etc. Here's one.
A loom machine to produce usable weaved cloths&nbsp;— pictures, video.
Pheromone Labs is using LEGO Mindstorms 2.0 to perform automated tests of a tablet app&nbsp;— link, video.
Low-cost nanoscope at UCL (news, main page)
Handling insect specimens (link)
UV-Vis spectrometer (Link)
Braille Printer (Link)