A freeware tool I use many times in IT is System Info for Windows.  This small lightweight utility comes packed with a lot of power.  No need to install anything on the local hard drive … just run it from a jump drive or pop it on a disc.  If you need detail about a computer you're working on, SIW will give you everything including the kitchen sink!

Showing all the applications installed on this computer.

Quickly scan your network to find the clients currently connected with IP information and MAC address.

One of the cooler features I use a lot is finding the BIOS information without having to go into setup.

With this application you can find out basically anything you want to know about your system.  Also, there are a lot of handy network monitoring tools as well.