I got into distributed computing with SETI in the heady mid-90's, long before BOINC - and have been addicted and crunching every since! It's amazing to me the wide variety of projects that are out there, but the ones that have really caught my attention recently are the non-CPU intensive ones that do various things.
The most interesting one that I recently joined is the Quake Catchers Network (QCN). You send them $50 and they send you back a USB accelerometer. I mounted a block to my bare cement basement floor, loaded up the app and bam - I'm now part of a growing earthquake monitoring grid! Obviously you want to put the sensor in the quietest most vibration free part of your home as possible. I almost mounted mine under my stairs since that's where quite a few of my computers are, then I thought better of it ;)
Having a computer that's on all the time shouldn't be an issue for members of PrimeGrid. Having one lower in your house might be a challenge; however it's not a requirement. Even if you are in an apartment on the 30th floor they can still use you (and when you register your computer height is one of the factors you can register) so don't let my OCD basement mounting sway you from at least looking into it.
Anyone else have a similar sensor/monitoring project that they participate in? Other than the $50 for the sensor, there was absolutely minimal effort to join this one and it's kind of interesting to peek in from time to time and see what it reports. I just wish there was a way with BOINC to force the BOINC screensaver to favor just one project; then again their could be I just didn't look. Hmm - time to google...