Sector antennas are pretty interesting. Unlike a normal directional, the beamspread on these comes in 90degree or 180degree. You usually see stuff like this on cell towers. These antennas are good for areas where you have 360degree coverage and dont want all your traffic coming in from one interface.
See also slotted waveguides as in http://trevormarshall.com/waveguides.htm and SlottedWaveguide here on the wiki.
