Kingdom of Loathing is a hilarious browser-based MMORPG that Aysmmetric Publications launched in 2003.
But browser-based entertainment is a great diversion, especially when you're at school or work and have a free minute to kill.
The majority of free MMORPGs are browser-based, meaning they play directly in your browser instead of operating in their own client.
Another favorite of mine, Hobo Wars is a browser-based MMORPG that puts you in the role of a homeless person trying to make it in the city.
Finally, there are purely browser based games, where the user interacts with forms to control the outcome of the game.