count uniques cells that match antoher cell value

Like so:

http://screencast.com/t/AcfDEO2uda

The array formula entered in F3 is:

=SUM(INDEX(($A$1:$A$8=E3)*(MATCH($B$1:$B$8,$B$1:$B$8,0)=ROW($A$1:$A$8)),0))

...confirmed by pressing CTRL+SHIFT+ENTER to activate the array, not just ENTER. You will know the array is active when you see curly braces { } appear around your formula. If you do not CTRL+SHIFT+ENTER you will get an error or a clearly incorrect answer. Press F2 on that cell and try again.

Then copy F3 down.