Over the years, I have played many, many games on the Internet. Here is a list of my five most-played games, and a little blurb about them.
#1 - Desktop Tower Defense. This game is a classic tower defense game - bad guys try to get across the map, and you have to stop them by building various towers. This one is number one for me because you can change the path of the creeps by changing your tower placement.
#2: Eyezmaze. I have no idea what this site exactly is, but it has a few puzzle games that are extremely entertaining. Try to place the objects in the correct order to win.
#3: Robot Unicorn Attack. Aaron sent this one to me. Guide a robot unicorn across the land. Listen to with sound on.
#4: Dino Run. This game is similar to Robot Unicorn Attack - guide a dinosaur across the landscape to escape extinction.
#5: Pandemic 2. You try to create a disease that will wipe out humanity. Somewhat morbid, but fun to see how diseases spread across the world.
Only try these if you have spare time!