I've used SpiderOak for about 10 months now, and it's the only cloud backup service that I trust won't be handing my data over to anyone. Everything encrypts client-side, and they (supposedly) have no way of accessing this data even if forced. It's basically online backup for those paranoid about privacy.
The client does need several improvements, but overall I'm quite pleased. The company offers extra free permanent storage to those who refer others, email with a special promotional code, or participate in online "tests". My favorite was their context for System Administrator Appreciation Day, where you had to fix a broken system and were rewarded with storage space. The contest is over, but the challenge is still up: https://sotss.spideroak.com/
(It was originally won if you managed to get the right program running, and it would ask for your username for the reward space. This doesn't seem to happen anymore.)
So far I've managed to get 29 GB of free space with them, which is permanent and accessible to even free users. So far, SpiderOak is the only cloud backup I've seen that has this "zero knowledge" service. If there are others, I'd love to hear about them.