GSmartControl is a graphical user interface for smartctl (from Smartmontools package), which is a tool for querying and controlling S.M.A.R.T. (Self-Monitoring, Analysis, and Reporting Technology) data on modern hard disk drives. It allows you to inspect the drive's S.M.A.R.T. data to determine its health, as well as run various tests on it.
Alexander Shaduri has done a decent job making a very useful GUI for an otherwise fairly cryptic S.M.A.R.T. attributes. When running GSmartControl you have helpful tips, color coded lines, help built into application, you name it... Few days ago it became available in Debian sid, but you can always visit the official GSmartControl page, download and build it for yourself.