If you travel at all, or are concerned about doing local backups, or maybe don't even have any backups today (talking primarily here about home and SOHO environments), then you've probably thought a bit about the various online "backup" (or "archive") providers. I've looked at a bunch of them and had mixed results. One thing to beware: many of these services style themselves "archive" solutions, rather than "backup" solutions, all to dodge reliability commitments. As such, I thought I'd share some of my thoughts/results...
There are a bunch of options, and I don't promise to have covered them all. Here are some that I've investigated and used.
Dropbox: This service was very good, but their recent issues with misinformation over the use of encryption, protection of files, potential sneak-n-peak disclosure to law enforcement, and strange Terms has caused me to ditch them. It's too bad, because I found them to be one of the best - if not the best - solutions out there. Unfortunately, they're not to be trusted. Not recommended.
SugarSync: I tried this service for a couple weeks and had a number of headaches. The mobile app kept abending photo uploads, leaving me unable to complete the process. I tried to sync a file between 2 devices and it "solved" the problem by creating multiple copies, and seems to have renamed the original file rather than just making copies. It was all a bit disconcerting. Not recommended.
SpiderOak: I've since switched to this service, and overall it's been decent. There is more I need to do in tinkering with it, including trying out the "sync" feature. That said, I've been very pleased with it, and have found the apps to be decent (mostly). One exception is the Android app, which seems to be a bit unstable. However, as I'm primarily using it for downloading a couple key files as needed, I think it's good. Recommended (register here).
A few recommended alternative approaches...
Google Docs: Most people are probably familiar with Google Docs by now. I've found it to be a handy alternative to the backup solutions out there. Also, with the addition of an Android app, I'm finding increasing uses for it. This will be a major part of my solution set as I try to move strongly toward a cloud-only presence (getting away from local data storage).
Evernote: If you've never tried out Evernote, then you really need to do so. I've only scratched the surface with it, but they have great device support, and provide a number of compelling features, including notes, screenshots, etc.
iCloud: It's too early to say definitively that this will be good, but I'm sure hoping it is. I'd love nothing more than to put my entire iTunes collection into the cloud so that it can be accessed by all my devices. This would be even better if Apple decided to support Android! :) (ha!) At any rate, I'm very hopeful that I'll be able to sync my entire iTunes collection into the cloud. It'll be worth $$ to me to do that, too.