wget - download files from remote servers, HTTP/HTTPS and FTP
history - see all the commands typed so far.
dos2unix - clean files edited in MS Windows notepad for Unix (see also unix2dos).
echo - print a message (useful in scripts).
nl - number lines of files.
sort - sort the lines in stdin, and print the result to stdout.
uniq - read from stdin and print unique (that are different from the adjacent line) to stdout.
join - joins two sorted files on a common key (the first column by default).
split - split a file into pieces.
cat - read lines from stdin (and more files), and concatenate them to stdout.
paste - joins files together in a column-wise fashion (in contrast to cat which joins files vertically).
sed - stream editor (search/replace values in a string).
tr - translate characters in a string using provided rules.
diff - compares files, and shows where they differ.
ln - make links and symlinks to files and directories.
gzip - compresses files, so that they take up much less space. It produces files with the ending .gz.
gunzip - uncompresses files compressed by gzip.
gzcat - lets you look at a gzipped file without actually having to gunzip it (same as gunzip -c).
pwd - tells you where you currently are.
w - shows who is logged in, and what they're doing.
who - shows who is logged on.
whoami - print effective userid.
id - print real and effective user and group IDs.
which - locate a command.
hostname - show or set server hostname
groups - show which groups user belongs to.
passwd - change user password.
htop - interactive process viewer (sometimes described as top on steroids). :)
ps - report a snapshot of the current processes.
kill - kills (ends) the processes with the spefified ID (to get the ID you can use ps).
df - report file system disk space usage.
du - estimate file space usage.
last - shows a listing of last logged in users.
git - a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
tree - list contents of directories in a tree-like format.
tmux - terminal multiplexer (similar to screen)
tee - read from standard input and write to standard output and files.