Ubuntu 9.10 (Karmic Koala)

There is a Drupal package in Ubuntu, so Drupal install on Ubuntu is just this line
sudo apt-get install drupal6

For the actual state of the Drupal packages in Ubuntu repository, check this site:
https://launchpad.net/ubuntu/+source/drupal6
The Debian repository is: http://packages.qa.debian.org/d/drupal6.html

The Drupal libraries are placed in the file system, as used to be in Linux:

1. /usr/share/drupal6

Most of the Drupal staff is here, the rest are symlinked to this directory, so it's the Drupal root directory

2. /var/lib/drupal6

The /files directory

3. /etc/drupal/6

The .htacces, /sites and /profiles directories

4. /var/www

A symlink is here, pointing to Drupal root (/usr/share/drupal6)

As this directory layout structure is not the recommended one from the Drupal community, use this install scenario just for testing Drupal, but please note: any updates will be difficult. See: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=526970

Or use Drupal in virtual machines, and it's a quick way to get Drupal up and running.

Then - at the time of writing this post - some more steps are necessary to get Drupal running:

1. enable the rewrite module and restart Apache:
sudo a2enmod rewrite
sudo /etc/init.d/apache2 restart
2. change the rewrite base in /etc/drupal/6/htaccess

from this one:
# RewriteBase /drupal
to this:
RewriteBase /drupal6
sudo sed -i 's/# RewriteBase \/drupal/ RewriteBase \/drupal6/' /etc/drupal/6/htaccess

3. optional: increase PHP memory:

sudo sed -i 's/memory_limit = 16M/memory_limit = 64M/' /etc/php5/apache2/php.ini

4. optional: if you want to access Drupal over the network, then edit your DNS settings or add this line to /etc/hosts on your client:
sudo su
echo "192.168.2.10 www.example.com" >> /etc/hosts

where change 192.168.2.10 to the IP address of your Drupal machine
5. open your browser and start the install process of Drupal:

if you are on the same host then:
http://127.0.0.1/drupal6/install.php
if you have done the previous point then:
http://www.example.com/drupal6/install.php