I use win32diskimager – http://sourceforge.net/projects/win32diskimager/
The only downside is an image is the full size of your card, regardless of how much is on it. I use an 8 GB sd card for Raspbmc and for Retropie, and they are nowhere near full, but an image is still 8 GB. The good news is, once that image is made you can zip it with 7zip or whatever your preferred archiving program is and it will only take up a fraction, because it can compress all that unused space in the image. My last backup of Raspbmc zipped is only 669 MB, but unzipped is 8 GB. I hate having to rescan all my movies and shows, this helps a lot! Hope it helps you!