The text-mode offers wide variety of system tools you can use. These tools are of course available in the graphical user interface as well, you just have to run them in terminal window such as xterm. Here are some of the tools which you may find most useful:

Archival utilities
gzip and gunzipReduces the size of the named files using Lempel-Ziv coding
bzip2 and bunzip2Compresses files using the Burrows-Wheeler block sorting text compression algorithm
xz and unxzCompression utility based on the LZMA algorithm ned by Igor Pavlov as part of 7-Zip
zip and unzipCompression and file packaging utility compatible with PKZIP (Phil Katz's ZIP)
cpio and tarPrograms to manage archives of files
Disk utilities
fdiskPartition table manipulator for Linux, which supports only MBR (Master Boot Record) partitions
hdparam and sdparamUtility for listing and potentially changing ATA/IDE and SCSI disk parameters
mdadmUtility program for creating, managing, and monitoring Linux MD (Software RAID) devices
Hardware monitoring tools
dmidecodeDumps computer's DMI table contents. This table contains a description of the system's hardware components
smartctlControls and monitors storage devices using the Self-Monitoring, Analysis and Reporting Technology (S.M.A.R.T.)
Filesystem utilities
mkfsTool to build a Linux or other supported filesystem, such as: ext2/3/4, reiserfs, btrfs, xfs, jfs, ntfs, fat, and more
fsckUtility to check and repair the supported filesystems (see above)
tune2fsAllows the adjustment of various tunable filesystem parameters on Linux ext2, ext3, or ext4 filesystems
CD ripping/burning software
genisoimagecreate ISO9660/HFS/Joliet CD-ROM images (so called .iso files)
System utilities
lsofLists information about files that are open by the processes running on the system
htopprocess viewer that is similar to the well-known "top" program, but provides much more features
Data recovery tools
ddrescueCopies data from a file or device to another, trying hard to rescue data in case of read errors
rsyncRemote file sync program
Networking tools
ipShows and manipulates routing, devices, policy routing and tunnels
ncUtility which reads and writes data across network connections, using TCP or UDP protocol
networkctlIntrospect the state of network links
Windows shares support
mount -t cifscommand to mount a windows share so it appears like a local filesystem
Other tools
sshOpenSSH client
service ssh startOpenSSH server
mcMidnight commander file manager which includes FTP/SCP client, hex viewer and text editor
aptApplication Packaging Tool to install additional software

xtermTerminal program (command line GUI)
chromiumInternet web browser and video player (runs as guest)
leafpadSimple text editor
qalculatePowerfull calculator
scrotScreenshot capturing program, press PrintScreen button to invoke
fehSimple image viewer and background setter