# Find CPU specifications
cat /proc/cpuinfo
# What pci cards are installed and what irq/port is used
cat /proc/pci
# Memory information
free
# How is the hard drive partitioned
fdisk /dev/hdXX -l
# How much free drive space
df -h
# Find running kernel version
uname -r
# Find X server version
X -showconfig
# What is the distribution
cat /etc/.product
cat /etc/.issue
cat /etc/issue
cat /etc/issue.net
sysinfo
# For finding or locating files
find
locate
which
whereis
Look at man <command> or info <command> for the flags and options you can use for bash commands.

What have I overlooked? Add your essential informational bash commands/scripts.