There is no software I know of that will (reliably) image a running server: For an image backup to be viable you really need the machine to be quiescent (no changes being made to the disks you're imaging). Failure to quiesce the system may result in unreliable/unusable data in the images. If these images are going to be mission-critical, replace "may" with "Will. Maybe not the first time, or the second, but definitely the one time you NEEDED it to work."
That said, you can boot into a recovery disk or LiveCD and use
dd piped over SSH, or look into something like g4u to do the job with a little more polish. If you're imaging with the intent to virtualize, your virtualization platform provider probably has tools specifically designed for this task.
If you have LVM set up you may be able to do the same thing with LVM snapshots, but the reliability is slightly lower than taking an image of a system that has been properly shut down (you may kick off the snapshot in the middle of a database write for example).
Note that while taking full-disk images is a valid backup method it is insanely time-consuming and difficult to manage, and taking the system to a state where you can do the backup usually entails an outage. If you want backups (something that you do periodically to guard against data loss) you are almost certainly better off investigating backup software - I suggest Bacula for *NIX environments.