The following is a non-comprehensive list of commands that I use regularly and had some difficulty finding when I first started with Linux. These commands are the fundamentals -- absolute basics.
execute a program
./
change directory
cd
go to home dir
cd <space>
change permissions
chmod
change ownership
chown
create a new user
adduser
change password
passwd
make an ssh connection
ssh -l <username> <address>
use text browser
lynx <url>
switch users
su -u <username>
remote root login
su -u root (after regular connection is made)
create multiple screens
screen
check connections
netstat
access a cd
mount /mnt/cdrom
eject a cd
umount /mnt/cdrom eject
access irc channel
irc (assuming it is in a directory that is included in your env path)
access your ipchains or iptables
./ipchains or ipchains (may have to be in directory depending on env path)
check your location
pwd
send network message
talk <username or ip>
check network config
linuxconf
find a file
find / -name <filename> -print (wildcards * can be used)
get info on a program
make a new directory
remove a directory
list all files in a directory
display file content
copy files
move files
search for a keyword
create a new text doc
start X client
stop a process
man <program name>
mkdir <directory name>
rmdir <directory name>
ls
cat <filename>
cp <filename>
mv <source file directory> <new file directory>
grep <keyword><filename>
pico or vi (if you are a newbie, use pico)
startx
kill <process number>
basic linux commands 
Linear Mode
