Kernel version and information:

lsb_release -a  -   E.g.  Distributor ID: Ubuntu    Description: Ubuntu 8.04.4 LTS Release: 8.04 Codename: hardy

Graphics card:
glxinfo – details about OpenGL, the Xserver, and your graphics card
glxinfo | grep direct – do you have direct 3d rendering?
glxinfo | grep vendor – graphics card vendor
lspci | grep VGA – specific graphics card model
glxgears – a simple 3d benchmark, prints frame rate to the terminal
xrandr – supported display resolutions

Audio:
lspci | grep Audio – audio controller
aplay --list-devices – more audio device information

Software versions:
cat /etc/issue – current distribution and version
apt-cache showpkg packagename – packagename’s version and dependencies
uname -r - Linux kernel version
uname -a – all kernel details

Networking:
lspci | grep Ethernet – Ethernet controllers
ifconfig – networking interfaces, IP addresses, and more

Processor:
cat /proc/cpuinfo – all processors, clock speeds, flags, and more
cat /proc/loadavg – processor load average for the last 1, 5, and 15 minutes
top – press C key to sort processes by CPU usage

Memory:
cat /proc/meminfo – amount of RAM and swap, and how much is being used for what
free -m – total, used, and free memory shown in MB
top – press M key to sort processes by memory usage

Hard disks:
df -H – partitions, as well as their mount-points and usage in GB
sudo fdisk -l – all partitions, their device names, and positions on disk

USB devices:
lsusb – USB buses and attached devices

Even more:
lshal -m – monitor for hardware changes
lspci – all PCI devices
hwinfo --short (install from package hwinfo) – overview of all hardware, as well as more detailed info
lshw – another program for listing hardware
lshw -html | w3m -T text/html – lists hardware with HTML output in the w3m web browser
uptime – current time elapsed since last reboot, users, and load average

http://tombuntu.com/index.php/2008/01/31/dig-up-system-information-using-the-terminal/