If you have not played an online flash game before, then you're missing out on a truly fun experience. There are sites with hundreds of addictive, fast-paced, and creative little games. You can spend hours stuck on just one game, determined to get to the last level. If you've got work to do, then you should probably stay away from flash games. You'll find yourself using valuable work time playing the games, and you'll have trouble tearing yourself away.
In order to find the most fun and addictive flash games, you can look in many different places. Some sites collect flash games, categorize them, organize them, and allow users to rate them. You can then see the most popular games, sorted by their ratings, the number of play, or the style of the game. This really helps to narrow down the type of game you play. Some days you might be in the mood for a fast-paced action game, or you may be in the mood for a logic game that requires you to think. All styles of games exist, it's just a matter of finding them. Some of the most popular game sites include Ebaum's World, Newgrounds, AddictingGames, and FlashArcade.
There are other sites which are focused more on the developers of free flash games. GotoAndPlay is a popular site like this, along with the Experimental Gameplay Project. While these sites are geared towards developers, there is still a place for people who just want to play games. In fact, by playing a game and telling the maker what you think, you can play a crucial role in the development of the game, and help to improve it for future players. So if you're willing to type a little bit to the programmer, those sites may be great for you.
If you're interested in making free flash games, then you need the program that gets it done: Adobe Flash. It's expensive to buy, but you can use a trial version for free. It also takes a lot of effort to learn 'however, that effort is not all in memorizing code and functions, as with other programming languages. Mostly you focus on learning the layout of Flash, and how to time things right.
Free flash games are all over the internet, and are extremely popular because of how fast and convenient they are. No file downloads, no installations, and no OS-specific games. So look up some flash games, and start playing!