Any web page can host a Unity web player, so if you have a web site, you can set show your web player by copying the HTML and .unity3d files generated by the build process.
See the Unity manual section on web player deployment.
are game portals that exclusively host Unity webplayers.
Some Flash game portals now support Unity, including
GameJolt and Kongregate are the most polished in terms of presentation and self-publishing convenience, but also have the most games, so you tend to get the most traffic shortly after first publishing your game.
facilitates setting up your web player on Facebook,
originally as individual Facebook apps
but now as part of a single Facebook app called dimeRocker arcade.
See the DimeRocker developer site
For comparison, you can view HyperBowl on
the HyperBowl web site,