Various virtual files in the
/proc filesystem contain a fair bit of info in human readable format, for instance:
cat /proc/cpuinfo for cpu details
cat /proc/meminfo for amount of RAM and what the kernel is doing with it (though
free -m does the same thing if you don't need as much detail)
Also other things like:
sudo dmidecode | less for a chunk of fairly detailed hardware info
sudo lshw | less similar but with more formatting options (see
man lshw for more hints)
There are several GUI utilities available too.
sysinfo for instance, which is not installed by default but is in the standard repositories so install with
sudo aptitude install sysinfo (or find it in the GUI package manager) and then run
sysinfo to call it into action. There is also a graphical interface for
lshw which shows much more detail but in a slightly less accessible way - run
sudo aptitude install lshw-gtk (or use the GUI package manager) to install and
sudo lshw-gtk to run.