This one's hung around an idea so simple, it's genius. It can automatically make a clone of any file or folder to somewhere else either at regular intervals or every time a change is made.
It doesn't perform some seven-hour operation at 4am on every second Thursday of the month, it doesn't litter your storage devices with weird proprietary formats, it doesn't waffle on about master boot records - it just makes an exact copy of the files you deem most important. A second copy, if you will.
This is a two-edged sword. The positive edge is that the headaches usually associated with back-up applications, for anyone who's either not entirely comfortable with complicated software or just craves convenience, are essentially wiped out. If a file becomes corrupted or deleted, or you make some changes you later wish you hadn't, just go open its cloned version from your other drive.
You can also create multiple profiles. So, for instance, it's checking to see if there's anything new to back up in My Documents every few minutes, it's making a copy of your Program Files folder once every Sunday, and it's taking a clone of expenses.xls every time a change is made to the original. It can also upload its copies to FTP, so they'll be in a secure place if your hard drive fails.
The negative edge is that Second Copy isn't much cop for making complete system backups. It can take a copy of everything on your PC, but this takes some time and slows down the system, and essential Windows data that isn't stored in simple files isn't backed up, so it's no good for restoring an entire OS. That isn't what it's designed for, though.
Yet for making sure, say, My Documents never comes to harm, it's got everything else beat.