I use Dropbox for replicating basically-public stuff between my two main computers, and it works great.
For backups, I use
CrashPlan with an unlimited family plan that was cheap after Mozy raised their prices. My backups are encrypted locally (with a key they don't have access to), they work whether I'm at home or on the road (unlike Time Machine), and I was able to seed an initial backup (1tb from three machines) over FedEx instead of my residential connection. It's already saved me once: I had a hard drive die while I was a long way from Miami, ordered a replacement drive, and was back up and running with the photos I took on the trip waiting in CrashPlan hours after I got home.
It works fine on my three Macs, not sure how the Windows or Linux clients are.