Alot of people have no idea about DC... (Distributed computing)
Basically what it does is someone creates a project (hopefully a "useful" one) which requires alot of computing power to solve. Now hopefully there is no way that the project could be funded through goverment or university grants, and the project doesn't benifit industry directly (Or the industry should pay for it itself).
So you have this huge project, it's divided up into workunits which are DISTRIBUTED to volunteer computers around the world. You decide to install their program and accept their task, the task 1 workunit will process on your computer for a length of time. Once the COMPUTING is done your computer sends back the result to the project.
www.seventeenorbust.com for example.
you install the client, download a k/n pair and check to see if k X 2^n + 1 = a prime number.
After a few days of processing your computer will either tell the project server yes it's prime or more likely no it's not prime.
Projects like this have been going on for years, the major drive is personal satisfaction, such as badges in BF2 or stats accumulation.
There are numerous projects out there like folding which is another good one, it basicaly models protein folding in an effort to understand drugs illness cancers etc etc etc. They have done some good work and published some of their data. Seti a very old project looks for Aliens buy analysising background noise looking for advanced intelligence (Or so they say)...
Personally I like the math ones b/c isn't totally computer based and the results if not useful are still certainly valid and push the limits of computing, data security, encription etc. A good example is MD5 the checksum used in banking, the project started and finished in a few days. This result basically shows that encription we were using on the internet was out dated. Another good one that also only lasted a few days, was decifering german sub transmission from WW2, almost all the transmissions were cracked in a few weeks and the project ended.