Name: libguestfs
Homepage: http://libguestfs.org/
Why: "libguestfs is a set of tools for accessing and modifying virtual machine (VM) disk images. You can use this for viewing and editing files inside guests, scripting changes to VMs, monitoring disk used/free statistics, creating guests, P2V, V2V, performing backups, cloning VMs, building VMs, formatting disks, resizing disks, and much more."
Open source: Yes
Source: http://download.libguestfs.org/1.38-stable/libguestfs-1.38.0.tar.gz