How do you turn on "windowed fullscreen" in TF2 and other Source games?
Solution 1:
Use the following as your command line: (From the library right click on the game, then pick Properties → Set Launch Options.)
-sw -noborder
-sw means "Start Windowed." -noborder tells Windows to not draw borders (titlebar, window buttons) around the window.
You can also designate screen size (e.g. -w 640 -h 480), skip the intro video (-novideo), turn on low violence mode in L4D (-lv) and other options that may or not mean anything to you. A fuller list can be found here.