I have been using sync.com for about a year now and am very happy with it. It says it's zero knowledge (I'm not qualified to judge the veracity of that) and costs under €60 pa for 500GB. I have also found them exceptionally friendly to deal with. It's ridiculous but I was chuffed to receive a postcard signed by about a dozen people after signing up. I used to use crashplan but after doing a successful restore from a supposedly good archive I found that thousands of files were missing. The other thing I use as was mentioned elsewhere in this thread is duplicati 2. Has worked perfectly for me so far.