How can I make ascii-banners from the command line?

I've always preferred figlet for big writing. Something about it preserves the character shape better. A bit subjective but there you go. Simple enough:

$ sudo apt-get install figlet
$ figlet oh hai!
       _       _           _ _ 
  ___ | |__   | |__   __ _(_) |
 / _ \| '_ \  | '_ \ / _` | | |
| (_) | | | | | | | | (_| | |_|
 \___/|_| |_| |_| |_|\__,_|_(_)

(It looks better in a terminal than it does here)

There are a ton of formatting options too that make it suitable for lots of different situations. It does this through "fonts" (standard, slant, shadow, small, smslant, bubble, digital, mini, etc). man figlet has a full listing of available styles and formatting options but here are a few examples:

$ figlet -f slant Hooah!
    __  __                  __    __
   / / / /___  ____  ____ _/ /_  / /
  / /_/ / __ \/ __ \/ __ `/ __ \/ / 
 / __  / /_/ / /_/ / /_/ / / / /_/  
/_/ /_/\____/\____/\__,_/_/ /_(_)   

$ figlet -f smslant Hooah!
   __ __               __   __
  / // /__  ___  ___ _/ /  / /
 / _  / _ \/ _ \/ _ `/ _ \/_/ 
/_//_/\___/\___/\_,_/_//_(_)  

$ figlet -f bubble Hooah!
  _   _   _   _   _   _  
 / \ / \ / \ / \ / \ / \ 
( H | o | o | a | h | ! )
 \_/ \_/ \_/ \_/ \_/ \_/ 

$ figlet -f mini Hooah!

|_| _  _  _.|_ | 
| |(_)(_)(_|| |o 

You need to install it before you can use it. Type in the terminal:

sudo apt-get install sysvbanner

This Package is not available in the standard installation and for this is why you have to install it manually.