Clicky Gone allows you to hide your selected windows from view, this includes the taskbar button associated with the application. This utility particularly helps with protecting your privacy by hiding things like your browser with banking details up and someone walks in on you.
A small program to show and hide windows in X window system. Most Linux distributions use a display server with X window system, you can easily hide and show windows using this program.
mapmywindows vs Hide my Windows opinions
It is directly inspired by this program and has similar functionality. It aims to provide a good alternative for linux (and potentially other unix users)
HideWin sits in the system tray and enables you to hide any top-level window with its popup tray menu or an ALT+letter hotkey that it assigns automatically. It appears to be abandonware, but it is only 13KB and still works.
The official website is no longer available. Last version, 1.0, released on June 13th, 2008, can be still downloaded from Softpedia.