ubuntu application to show calendar with week numbers

The default time & calendar indicator applet has this built it - you just need to enable the option.

Running gsettings set org.gnome.shell.calendar show-weekdate true solved the problem for me.

On 12.04 after moving from unity to gnome-shell, I had to

gsettings set com.canonical.indicator.datetime show-week-numbers true

to have weeknumbers in the calendar notification applet.