Acronym for United Devices, a company/program that works much like SETI@home and distributed.net using the principal of distributed computing. Once you have downloaded their client from www.ud.com, you can set up a user name and download a small processing unit from a company that has paid UD for their services. Your computer then processes the unit (doing various tasks, although the only one so far has to do with comparing proteins). Soon to be added is a project helping cancer research.
One incentive is the possibility of winning a cash prize or other gift based on the amount of help you provide them. The average user never wins, but it is still exciting just to participate in a distributed computing project. Also, you are able to install the client on multiple computers under the same user name, so as to increase the work (and chance of winning) that you get credited for.