UrBackup Server runs on GNU/Linux, Windows and FreeBSD.
There are some considerations however if you want advanced features like compression or block level deduplication.
If you want the backups to be compressed the underlying file system has to do that for UrBackup. In most cases this means the operating system has to support that. Since Windows Server 2003/XP Windows has, with NTFS, compression capability.
In Linux following filesystems support compression:
The backup file system has to be able to handle hard links and symbolic links. NTFS is able to handle both (Symbolic links (junctions) are supported since Windows Vista). All Linux/Unix file systems support hard links and symbolic links. FAT does not support this.
The client images are saved into one large file. The file system has to be able to handle them. On Windows NTFS supports very large files. You will not have any problems there. On Linux you have to enable the large file support in the kernel and for the specific file system. Nowadays this is enabled per default. Then the maximum file size depends on the used file system. With ext3 it can be as small as 16GB if the drive is formatted with a small block size. Usually it is 2TB though (with standard block size).
With deduplication data blocks which are the same in different files are saved only once on the hard disk. ZFS is able to do that. There are also professional storage systems which do deduplication - sometimes in hardware. Btrfs does support offline deduplication. There are some other fuse file systems which support deduplication but those do not seem to be stable or performant. If ZFS is chosen you should put enough RAM into the backup server otherwise deduplication will cause a huge slowdown. UrBackup can be configured to accommodate a slow deduplicated backup storage. Another option is to use the Windows Server offline deduplication (and compression) available since Windows Server 2012.
UrBackup can also use btrfs in a way that makes deduplication sometimes unnecessary. See here.
UrBackup Client runs on Linux, Windows and Mac OS X.
Image Backups only work with a Windows client and can only be done with volumes which are formatted with NTFS.