Folding@Home is:

"Folding@home is a distributed computing project -- people from throughout the world download and run software to band together to make one of the largest supercomputers in the world. Every computer takes the project closer to our goals. Folding@home uses novel computational methods coupled to distributed computing, to simulate problems millions of times more challenging than previously achieved."

This project is allowing biologists to fold proteins. I am no biology expert, but in English, it will help to eliminate some diseases.

EDIT: In comments, some people do not understand what this program does. It uses your computer to help them fold proteins rather than having to buy new computers to do the same thing. This helps them because they might buy 3 computers to help them at their end, but people allow them to use their computer to do calculations. This adds more computers to the project.

Step 1: Download the Client

First, visit the website folding.stanford.edu and click download. Then, based on information sent by the browser, it will determine the best client for you. I have Windows XP, so it recommends that to me. It has clients for Mac and Linux.

Step 2: Installing the Client

Once you have download the file (I downloaded the graphical one), run the setup file. Follow the on screen prompts to install the client. It has a description, a license (GNU GPL), and user options for what you want your username to be.

When it finishes installing, uncheck the "View ReadMe" and then click finish.

Step 3: Use the Program

Now that you have finished, you will have a new System Tray icon (it is highlighted below) and if you double click on that, you get the following window.

Step 4: Let the Program Run.

All you have to do now is let your computer run. If you want and have Windows, go to Start-> Shut Down -> Stand By. It puts the computer in a low power state but allows the program to run at full.

Step 5: Changing Your Username and Team Number

I added this step since it is important to know how to do the things that it will teach you. First, (I am assuming you are using the graphical one since that requires manual configuration, the console can be done when you first start it.) right-click on the system icon in the bottom right of the screen. It is highlighted below. Then go to configure. In the box that pops up, change the options. Press ok.

Edit: I created a team, as suggested in comments. The team number (what you enter) is 129004. Make sure to not have the period.