BOINC is a widely used open source Desktop Grid environement. The main website can be found at http://boinc.berkeley.edu/ . Participating in a BOINC based Grid is easy. Just download the client for your computer from the main BOINC website. Setting up a BOINC based Grid is not easy. In fact it takes up to several months for a first Grid to get up and running. Upgrading a Grid is also a complex task as the focus of the BOINC project is developing new features and doing research, not about making good backwards compatible versions of the server software.
Hence, just like you have Linux and Linux distributions, one has several distributions of BOINC Grid server versions. The one we use is: http://desktopgrid.hu. This distribution quickly installs a basic BONIC server on a Debian Linux. You can be up and running within a day. Configuring the Grid then still takes a lot of time, but less than starting from scratch.
The technical information on BOINC is availble in the Wiki: http://boinc.berkeley.edu/trac/wiki/.