If you are using some variant of UNIX/Linux, there is something called "Process Accounting". It is capable of logging every process, and does so when the process terminates.

It actually comes with RedHat 7.1 (I don't know about previous versions), and the package is called psacct.. so you can just do a

rpm -qa|grep psacct
[download]

to see if it's installed.

As far as documentation goes, you can try:

man accton
man acct
man sa
[download]

And to keep things on topic for this site, by all means, you could use a perl script to dig through the log file to generate usage information and statistics for you if 'sa' doesn't do what you are looking for :-)

As a side note -- process accounting is often something used on only the most secure of sites. Not all users would be fond of having their every command logged. It can border on being Big Brother. However, I'm not saying you shouldn't do it, and there are definitely times where it is useful or necessary.
-Eric