There seems to be a nice initiative, "world community grid". I think, IBM sponsors this community computing grid. I have been participating on this grid, since quite a few days now, and it really works! and I believe, it does make a difference to community good.
This grid is composed by, computers which could be normal public personal computers at home, or office or any kind of computers that all can connect to the web. When a grid client is connected to the web, enabled by user authentication, the client computer participates in numerous public computing projects. Joining the grid, helps us to donate our computer's processing power to computations needed by these public projects, normally those that require massive computing simulations in short time.
Joining the grid, doesn't disrupt the normal user activity on client computers, and the grid client intelligently utilizes memory (a very less amount of memory is needed by the grid client, while it works, which is normally as less as 5-10 MB) and the CPU, without disrupting anything for user's personal activities. It is also possible to configure the user's grid activity, about how to use one's CPU. Somebody may want to work in the default mode, or can give more CPU usage to the grid project tasks. The default mode works, well for me.
All these details, and much more are available on the "world community grid", web page.