chmod +x <filename>
Makes a file runnable - chmod changes permissions of a file.
Run a file.
View file type and information.
Print all printable strings in a file.
nc <host> <port>
Use netcat to connect to a remote host. This is very common in the pwn category and other remote challenges.
View the hex dump of a file using xxd.
base64 -d <filename>
Decode a Base64-encoded file.
Browser developer tools
Found in most browsers by pressing [F12].
Here you can view the network traffic, cookies, page source code and more.
Most web pages has this file, located at www.example.com/robots.txt.
This is to prevent web crawlers from traversing certain places of a web page.
Hidden paths and information may be found here.
The Gnu debugger, built into most Linux systems.