How to close all open applications except TestComplete without knowing which applications are open?

One method...

You could use Sys.FindAll, iterate and evaluate the objects returned, and then .Terminate() the desired applications/processes.

I think it would be dangerous to do so...

anyway...

I use this delphi script in my librairies to kill a process of which I know the name :

procedure KillProcessus(NomProcessus: string);
var
  p, IsClosed;
begin
  p := Sys.FindChild('ProcessName', NomProcessus);
  while p.Exists do
  begin
    p.Close;
    IsClosed:= p.WaitProperty('Exists', False);
    if not IsClosed then
      p.Terminate;
    p:= Sys.FindChild('ProcessName', NomProcessus);
  end;
end;

you could even try knew processes or loop into all processes and exclude in the processes you want