My initial interest in SVA Software's D_back backup program was due its "back up your PC on insert" capabilities. There are plenty of programs that will back up a USB drive to your PC when it's inserted, but only a few that backup your PC to said USB drive as D_back ($37, 21/day free trial) will. The program has some other unique features and worked quite well once configured, but it's a bit rough around the edges and not particularly intuitive.
Why was I interested in the "backup on insert"? Because I've been doing it for years with external drives, using the autorun.inf file and batch files. Unfortunately, in the name of security, Windows 7 won't run the autorun.inf file. Now the only solution is a program such as D_back that sits in the background waiting for a drive to be inserted.
D_back's comprehensive scheduler, which facilitates the "on insert" backup, has a separate interface and may run either in the background or as a service. It's a bit tricky to set up an "on insert" backup, but there's a step-by-step tutorial on developer SVA-Software's Website. Thankfully, you may limit the backup to only a device with a specific volume label so that you don't back up to every portable storage device you insert--a trick I wish CrashPlan would emulate.
Another unusual and useful feature of D_back is backup via e-mail. The program zips up and attaches the backup to an e-mail, which it sends using SMTP. SMTP setup was problematic in version 1.69.8 that I first tested; however, 1.69.10 cured the problems. Obviously, you'll want to limit an e-mail backup to a small number of files--most inboxes don't allow more than a total of 10MB to 25MB in attachments. D_back also allows FTP and local backups and all backups may be incremental, differential, or stacked (multiple versions). You may also back up uncompressed for easy browsing and restore.
D_back has a few foibles, nearly all of which are interface-related. The nag screen on the demo gives you no hint on how to continue (hit the close button), and the default backup job backs up your program files folder, not My Documents or something more data-oriented. SMTP is spelled SMPT in one instance, and there are other signs that someone should spend some time fixing stuff minor stuff. Navigation in general is doable, but often unintuitive.
D_back is very versatile and once a backup is defined, it works very well. It's simply that setting up said backup isn't nearly as easy as it could be, and the program is a bit pricey. That said, its unusual abilities make it well worth checking out.