This is a good list.
There are three kinds of Linux commands:
commands I use frequently
commands I've never seen or don't know about. There's almost nothing in standard POSIX that falls in this category, and a lot of OSS that does. E.g., I use to always reach for fuser until I realized it's not a base install on many distros, so I switched to lsof which is is, and is also both more powerful and harder to use.
commands I've seen before but use so infrequently I forget they exist, or what they're called. This is sadly a larger set than I'd like.
Some of these in this list are the third kind.

Looks like the site is down or blocked in my country.

Could anyone please be so nice and copy paste those commands here?

Supervise command (run every 2s)

    
watch "ls -larth"

Kill program using one port

    
sudo fuser -k 8000/tcp

Limit memory usage for following commands

    
ulimit -Sv 1000       # 1000 KBs = 1 MB
ulimit -Sv unlimited  # Remove limit

Rename selected files using a regular expression

    
rename 's/\.bak$/.txt/' *.bak

Get full path of file

    
readlink -f file.txt

List contents of tar.gz and extract only one file

    
tar tf file.tgz
tar xf file.tgz static

List files by size

    
ls -lS

Nice trace route

    
mtr google.com

Find files tips

    
find . -size 20c             # By file size (20 bytes)
find . -name "*.gz" -delete  # Delete files
find . -exec echo {} \;      # One file by line
./file1
./file2
./file3
find . -exec echo {} \+      # All in the same line
./file1 ./file2 ./file3

Print text ad infinitum

    
yes
yes hello

Who is logged in?

Prepend line number

    
ls | nl

Grep with Perl like syntax (allows chars like \t)

    
grep -P "\t"

Cat backwards (starting from the end)

    
tac file

Check permissions of each directory to a file

It is useful to detect permissions errors, for example when configuring a web server.

    
namei -l /path/to/file.txt

Run command every time a file is modified

    
while inotifywait -e close_write document.tex
do
    make
done

Copy to clipboard

    
cat file.txt | xclip -selection clipboard

Spell and grammar check in Latex

    
detex file.tex | diction -bs

You may need to install the following: sudo apt-get install diction texlive-extra-utils.

Check resources' usage of command

    
/usr/bin/time -v ls

Randomize lines in file

    
cat file.txt | sort -R
cat file.txt | sort -R | head  # Pick a random sambple

Even better (suggested by xearl in Hacker news):

    
shuf file.txt

Keep program running after leaving SSH session

If the program doesn't need any interaction:

    
nohup ./script.sh &

If you need to enter some input manually and then want to leave:

    
./script.sh
<Type any input you want>
<Ctrl-Z>          # send process to sleep
jobs -l           # find out the job id
disown -h jobid   # disown job
bg                # continue running in the background

Of course, you can also use screen or tmux for this purpose.

Run a command for a limited time

    
timeout 10s ./script.sh

Restart every 30 minutes

    
while true; do timeout 30m ./script.sh; done

Combine lines from two sorted files

    
comm file1 file2

Prints these three columns:

        Lines unique to file1.
    Lines unique to file2.
    Lines both in file1 and file2.

With options -1, -2, -3, you can remove each of these columns.

Split long file in files with same number of lines

    
split -l LINES -d file.txt output_prefix

Flush swap partition

If a program eats too much memory, the swap can get filled with the rest of the memory and when you go back to normal, everything is slow. Just restart the swap partition to fix it:

    
sudo swapoff -a
sudo swapon -a

Fix ext4 file system with problems with its superblock

    
sudo fsck.ext4 -f -y /dev/sda1
sudo fsck.ext4 -v /dev/sda1
sudo mke2fs -n /dev/sda1
sudo e2fsck -n <first block number of previous list> /dev/sda1

Create empty file of given size

    
fallocate -l 1G test.img

Manipulate PDFs from the command line

To join, shuffle, select, etc. pdftk is a great tool:

    
pdftk *.pdf cat output all.pdf        # Join PDFs together
pdftk A=in.pdf cat A5 output out.pdf  # Extract page from PDF

You can also manipulate the content with cpdf:

    
cpdf -draft in.pdf -o out.pdf      # Remove images
cpdf -blacktext in.pdf -o out.pdf  # Convert all text to black color

Monitor the progress in terms of generated output

Write random data, encode it in base64 and monitor how fast it is being sent to /dev/null

    
cat /dev/urandom | base64 | pv -lbri2 > /dev/null

# pv options:
#   -l,  lines
#   -b,  total counter
#   -r,  show rate
#   -i2, refresh every 2 seconds

Find packages that have a given file in Ubuntu

    
apt-file update
apt-file search dir/file.h

Fantastic list. I just want add one to this sublist. I use these often:
Find files tips

find . -maxdepth 1 -type f -printf '%f\n'
-printf '%f\n': This is a function to print in a formatted manner, typically used in C or other languages. %f means here print the filename only, without directory or slashes. And add a newline off course, but you could also print it by space enclosed between quotes: "%f" I prefer this over executing separate echo process.
-type f: This will limit the output depending on the filetype you define. Here f means files only. This can have multiple types too, in example directories or executables.
-maxdepth 1: Do not search subdirectories, only look in current directory like ls does. You could specify more depth too. And there is even -mindepth 2 in example, if you want to skip some top level directories and only search somewhere deep. This makes sense if you have organized structure of directories in example.

Nice.
- ...and renice.

The most interesting command for the Linux shell is known as Barmin patch.

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