The list in the main BOINC site displays 47 different computation projects. According to their statistics page, BOINC has a total of 2,481,431 users (but only 264,863 active), with a combined computation power of 7,084,872.8 GigaFLOPS.
For a novice user, installing BOINC wouldn’t be that trivial. One has to sift through many versions, use the command line through installation (any computer with access control), and setting up a project also involves copying and pasting text.
I tried joining the GPUGrid project, which required me to check for specific hardware in a forum post, install CUDA (a command line task by itself). Having done all of that, the program is alright. Lots of numbers are displayed in normal software mode, but the screensaver is actually quite pleasing. Overall, I don’t think I would’ve survived this as a novice user.
Zooniverse has 15 projects, 12 active. As of now, they have 703,229 users active.
The Zooniverse site is great. It has a commercial website look, with good graphics and well-written copy. Installation is simple and joining a project is easy. I’ve had trouble running computations, but the software was nice enough to inform me that the server was down.