I've got SETI because it's beautiful and it makes me feel useful. Also my son is utterly convinced he can see it finding the aliens, which, on reflection, amuses me more than it probably should.
I've had it for years and years and I don't think I will ever get fed up with it.
I've got a zoomquilt screensaver, which is nice, but I didn't download it, I got it off a cover CD, so I can't give you a link. It's for OS X, too. I don't know if there's a Windows version.
I used SETI for a while, but it kept refusing to connect to the server, so I gave up. These days I'm using the windows 'my pictures' screensaver, mostly because I have quite alot of pictures that confuse my roommate.