DefragNT. Free defrag program for Windows XP etc.
Only OS-es built on the NT kernel newer than 3.5 are supported. FAT partition support for Windows NT4, Windows 2000, Windows XP and newer; NTFS partitions supported only beginning with Windows XP. The name of this utility might change to DefragXP.
A few other related free products I’ve come across over the years:
Diskeeper Lite
Buzzsaw & Dirms
PageDefrag
Contig
Nice Peter, thanks. :-)