As you can see these services are relatively similar, though might be utilized for different purposes. Mozy and Carbonite are direct competitors, as their pricing structure makes clear. Dropbox is used by most people for temporary storage, sharing or projects but if you don't have a ton of stuff on your hard drive it works just as well for the important stuff you need to keep. Backify is a relatively new company, but their pricing is hard to pass up. All of these companies use high-level encryption, extensive redundancies of their own systems, and physical security to keep data safe.
The most arduous part of using online backup is the start up time. The first time you make a backup you're copying your whole hard drive across the web, which can take a while even with the best internet connection. We suggest beginning the process and leaving your computer on overnight. Once you've got the initial backup complete, it's a simple matter of scheduling a time for the backup to occur. With Mozy and Carbonite's clients the backup can be scheduled automatically. Backify's client monitors your files and uploads any changes it finds, keeping up to 30 versions of a single file (similar to Apple's time machine). And Dropbox has a folder on your computer that, as long as you're working with files inside it, will automatically update.
External drives are not included with Carbonite's backup and if you backup with Mozy without your external hard drive connected it will mark those external files for deletion after 30 days. It must be said that these services are aimed at backup, not archiving. Complete restoration of your hard drive is also a long process that can take longer over the internet, so Mozy's DVD option is quite nice. Which service you choose will likely depend on how much space you have, how you want to use your backed-up data and how much you want to think about it.
Did we leave out your favorite back-up service or feature? Tell us in the comments.