Wipe your hard drive clean

Windows
If you're after a free solution, Runtime's DriveImage XML will fit the bill. Those who find more comfort in commercial solutions can't go past the excellent Acronis True Image Home 2010.

OS X
OS X has a disk image tool built in. Just go to your Applications/Utilities folder and run Disk Utility, select the disk or partition you wish to create an image of, and click the New Image button.

You can then view the contents of the image simply by double clicking on the file, or you can restore it entirely by clicking the Restore button in Disk Utility and following the instructions.

Linux
(Editor's note: depending on your set-up, you may need to sudo these commands) dd is the tool of choice here. For example, dd if=/dev/sda of=hdd.img will create an image file called hdd.img from the hard drive located at /dev/sda. Obviously you'll have to replace /dev/sda with the hard drive or partition of your choice.

In this command if represents the input file or device, whereas of represents the output file or device. If you've got multiple partitions on the one disk it can get tricky, so if you want to keep things easy, perhaps take an image of individual partitions rather than the whole disk.

To restore to another drive, just use dd again, but use the hdd.img file as the if argument, and your target drive as of.

If you'd rather not restore the entire image just to look at what's in it, you can mount the file to a folder to browse:

mount -r -o loop ./hdd.img /mnt/test

You should now see the contents of hdd.img in /mnt/test (assuming you've created the directory beforehand) — obviously you'll need to substitute in your own values for the image file and folder where you want to mount it. The -r argument to mount the image as read only is likely overkill, but we want to be safe.

To unmount, simply point umount at the directory the image is mounted in.

umount /mnt/test