I have a new favorite online game: Robokill.
It's a really fun top down shooter in the style of the old Berzerk, Frenzy or Robotron.
You move with keyboard and aim and fire with the mouse. The first four or five levels are free, and I got addicted enough to pay the $10 to play the rest of the game and get the more advanced weapons.
It's addictive and lots of fun.
Aside for the gameplay itself, I think my favorite part are the casing shells that stack up if you keep shooting in the same place.