BOINC is an open-source software platform for computing using volunteered resources
Berkeley has this really cool program called BOINC that you can download and donate your computer's resources to processing scientific data. There are a bunch of projects to pick, from working on climate change, to cancer, to the Large Hadron Collider.
One would assume they mean sitting around, doing nothing. Some would rather use some electricity to support a good cause than have the computing power sit there idle.
Now I know this is being done with encryption, but an open tunnel direct to your non dmz-ed system, is just begging to be hacked, and it will be, without a shadow of doubt.
As far as I know, the BOINC client just pulls down new data for processing when each batch of work is done. There's no pushing and no open tunnel or port. The software risk would be malicious code in a particular project (e.g. if it said it was folding proteins but actually mined bitcoins). I hope there's some vetting of project code.
The other risk is hardware (especially CPU and RAM) running its lifetime down more quickly because of the continual heavier usage.