sudo add-apt-repository ppa:kevin-mehall/libwnck
sudo apt-get update && sudo apt-get install libwnck22
And then restart the Gnome Panel:
killall gnome-panel
Then, right click a window in the "Window List" on your Gnome Panel and select "Always on visible workspace". That will "pin" the window to the taskbar (Gnome Panel).
The "pinned" windows will be displayed on all the workspaces, just like the Notification Area applications.
I have noticed a bug though: after pinning a few windows, the windows left unpinned take the whole space left on the Gnome Panel, unless you have some other applet to the right of the Window List. So if you only have 1-2 windows unpinned, they will be huge. Adding an applet to the right of the Window List can fix this by moving this applet more to the left, to adjust the Window List size.
More info (and credits for this "Pin tab" feature), on Kevin Mehall's blog.