Ubuntu – What software should I install for opening .stl files

For billing/timing, you might want to look at Cafepilot. It is free an open source software. It's currently aimed at people paying for their usage but you could give people tickets to use it (CP would think they're worth money but you'd just give them out for free).

For monitoring, remote-controlling and updating, check out Canonical's Landscape tool. It's currently quite expensive but might justify itself... You're always free to haggle with Canonical because your situation is somewhat different from the standard enterprise "seat" concept.

Otherwise you can loop together tools like Munin or Cacti with Puppet. Puppet is free and is in the repositories but you probably want to look at the documentation on their main site. Note there is also a paid-for "Enterprise" version of Puppet that has a slightly augmented feature set.

I like rawtherapee. You may want to give it a shot. There is also UFRaw, but I personally don't like it as much.

In terminal:

sudo apt-get install rawtherapee

and

sudo apt-get install ufraw

respectively.