KDiskMark is an HDD and SSD benchmark tool with a very friendly graphical user interface.
GParted Partition Editor
GParted is a free partition editor for graphically managing your disk partitions.
GNOME Disks
GNOME Disks, gnome-disk-image-mounter and gsd-disk-utility-notify are libraries and applications for dealing with storage devices.
Disk Usage Analyser
Disk Usage Analyzer is a graphical application to analyse disk usage in any Gnome environment.