Free Process Director is a very powerful tool which displays all the processes running on your PC, and tells you a great deal about what they're doing.
The program displays its processes in the form of a tree. Expand Explorer.exe, say, and you'll find further sections of the tree to display its loaded modules, memory use, open files, threads, and more. Which is useful, although the tree presentation isn't the easiest to use; we find simple tabbed dialogs, as you'll find in similar tools like Process Explorer and Process Hacker, easier to use.
Free Process Director gets much more interesting when you right-click individual processes, though, and discover some of the things it can do.
The "Monitor File Access" option will show you which files the process is opening, reading, writing and closing, for instance.
Choose "Monitor IP Traffic" and you'll see network connections being opened and closed, the source and destination IP address, even the User Buffer so you can see some of the data being sent. (Monitor an email client, say, and you'll see your account user name and password.)
There's even an option to "Monitor Process Execution", which in theory allows developers to watch Windows API use. Although this only worked occasionally for us.
And there are plenty of more basic, but still useful right-click options. So you can view the DLLs a process has loaded, or the files it's locked; a "Reclaim Memory" option cuts process RAM use to a minimum; and there are options to terminate unwanted processes, suspend others, and resume them later.