Flash games exist in a strange world. Anyone can make them. They're not bound by big budgets or proven formulas like their console cousins. Any motivated programmer or 14-year-old kid can make a game and get it featured on Newgrounds if it's good enough. Any time someone has a novel gameplay idea, everyone else wants to put their spin on it. As a result, some weird genres emerged. XBox games come in clear, simple genres like "sports" and "shooters," but flash genres are more along the lines of "games where you fling things with your mouse." These are the most popular game genres you can play in your Web browser.
All motorcycle physics games can be traced back to the old PC game Action Supercross (later Elastomania). It was a motocross game where you drove an elastic motorcycle through twisty levels, collecting apples and trying to reach the goal. Flash developers decided that elastic motorcycles and collecting apples were too much for the fickle Internet audience. They stripped it down to the basics. As a result there are around a thousand games where you reach the goal by pressing back to lean back, forward to lean forward, up to drive, and down to brake. Originality aside, there are a surprising amount of good motorcycle physics games. They're all exactly the same, but if you pretend each new game is a level pack you can play forever. They're not specific to motorcycles, either. Monster trucks, skateboards, bicycles and Mars rovers have all proven that they can lean, accelerate and brake.
Notable examples:TG Motocross 4 Trials Max Dirt Bike
The Little Big Planet of the Flash universe. I don't like playing line drawing games because I'm a terrible artist and I have no patience. I do like watching YouTube videos of crazy people playing line drawing games. The things people have created in Line Rider are baffling. There are serious works of art for that little sledder to traverse. Objectives in line drawing games range from "no objective" to "solve the puzzle," but it always boils down to whether or not you can make your own fun with the tools provided. Essentially, your own Flash Playground. In some versions you can control the person riding the bicycle in your playground, in others you're a meticulous God, creating a universe of angles for him to navigate by the will of gravity. Or you're me and can't get the guy to go five feet without falling on his head.
Notable examples:Line Rider Free Rider 2 Draw Play 3
Tower Defense may be the only game to spark it's own genre of the same name. Other games had opportunitiesMario could have been called "Platformer," Street Fighter could have been called "One on One Fireballs and Uppercuts Fight"but Tower Defense was the only one with a literal-enough title to pull it off. There are so many Tower Defense games, you'd almost think defending a tower would be a fun thing to do in real life. Think of a word, put it in front of tower defense and google it. It probably exists as a map or full stand-alone game. That's kind of the charm, though. Tower Defense games are fun enough and you can find one in any flavor you like. They're great for situations where you can't give your full attention to a game. Set up your line of defense then get back to taking notes in class while the computer mows down a wave of computer viruses, orcs or zombies for you. Then do it again times infinity.
Notable examples:Tower Defense Desktop Tower Defense Plants vs Zombies