Web games, also known as browser games, are games played over the web. Not to be confused with Freeware Games, which are distributed for free and act like regular computer programs. Web games are played directly through the web browser, often (but not always) with the help of a plugin.
Homestar Runner - A site famous for its web toons but also contains a number of popular games.
Jay is Games - Critical web-game reviews on countless titles (with links included to try things out for yourself). Also a source of succinct walkthroughs and development contests where designers are challenged to create original works on a loose theme, and rewarded by voting. Gamers are rewarded by awesome original games that are free-to-play online.
Kongregate - Home to an active community of developers and the gamers who provide feedback (and occasionally donations) to their works. A large part of the site is the social networking aspect of flash gaming, increased by meta-game achievements the developers and site mods help create. Recently made part of the Gamestop network.
Newgrounds - Known for its flash games, but also known for flash movies. Along with Kongregate, one of the leaders in web-gaming developer sites that include social review and meta-game rewards for players.