Linux

The first step when landing on host should be understanding who your against to - what OS, what process are running, what users exists and more, this can be done by looking at the following files (remember - in Linux everything is a file):

Distribution type:

cat /etc/*-release

Kernel version:

cat /proc/version uname -a

view if you can run anything as sudo: (check for GTFObins)

Sudo -l

Check common files:

cat /etc/profile 
cat /etc/bashrc 
cat ~/.bash_history 
cat ~/.bashrc 
cat ~/.bash_logout

What services running (filter by root):

ps aux

ps -ef

top

Check configuration files:

cat /etc/syslog.conf 
cat /etc/chttp.conf 
cat /etc/lighttpd.conf 
cat /etc/cups/cupsd.conf 
cat /etc/inetd.conf 
cat /etc/apache2/apache2.conf 
cat /etc/my.conf 
cat /etc/httpd/conf/httpd.conf 
cat /opt/lampp/etc/httpd.conf

Check local ports and what listens:

netstat -antup

View list of users:

cat /etc/passwd | cut -d: -f1

Search for ssh keys:

cat ~/.ssh/authorized_keys 
cat ~/.ssh/identity.pub 
cat ~/.ssh/identity 
cat ~/.ssh/id_rsa.pub 
cat ~/.ssh/id_rsa 
cat ~/.ssh/id_dsa.pub 
cat ~/.ssh/id_dsa 
cat /etc/ssh/ssh_config 
cat /etc/ssh/sshd_config 
cat /etc/ssh/ssh_host_dsa_key.pub 
cat /etc/ssh/ssh_host_dsa_key 
cat /etc/ssh/ssh_host_rsa_key.pub 
cat /etc/ssh/ssh_host_rsa_key 
cat /etc/ssh/ssh_host_key.pub 
cat /etc/ssh/ssh_host_key 

View crontabs

crontab -e root 
crontab -l 
ls -alh /var/spool/cron 
ls -al /etc/ | grep cron 
ls -al /etc/cron* 
cat /etc/cron* 
cat /etc/at.allow 
cat /etc/at.deny 
cat /etc/cron.allow 
cat /etc/cron.deny 
cat /etc/crontab 
cat /etc/anacrontab 
cat /var/spool/cron/crontabs/root 

Web servers files

ls -alhR /var/www/ 
ls -alhR /srv/www/htdocs/ 
ls -alhR /usr/local/www/apache22/data/ 
ls -alhR /opt/lampp/htdocs/ 
ls -alhR /var/www/html/