Option 1: xbindkeys & wmctrl
While It's not as user-friendly to setup and manage, you can get this kind of functionality in many environments, by using
Xbindkeys can set to start at login. It will monitor a file called
.xbindkeysrc, which can contain lines like this:
#Launch or switch to E-mail
"wmctrl -xa Thunderbird || thunderbird"
Alt + 2
#Launch or switch to Konsole
"wmctrl -xa Konsole || konsole"
Alt + 3
#Launch or switch to IRC client
"wmctrl -xa Xchat || xchat"
Alt + 4
Like Unity, with this recipe you set up a key to switch to an application if it is running, or launch it if is not. I used
Alt here, but you could use
Mod4 instead, I think.
Option 2: Use workspace-switching shortcuts
Something similar is to assign keyboard shortcuts that switch to specific desktops (or "Workspace"). If you run one application per workspace, then the shortcut effectively switches to that application. Look under
Keyboard: Shortcuts: Navigation for the place to set the workspace-switching shortcuts.
Option 3: gnome-shell-extensions-windows-navigator
After installing this Gnome extension, in overlay mode you can hold the ALT key and see a number assigned to each window. You can then press the number to switch the window. More here, including installation instructions.