Free games are not created equal. Some are better than others, of course.
If you don't have to pay anything, it might not seem to make much difference whether or not the game is of good quality, but that all depends on your objective. Your only goal might be to have some fun. Even the amount of fun that you have depends on the quality of the game.
In general, the game should be easy to use. The rules should be relatively easy to understand. Ideally, the difficulty will increase as you continue to play.
There are benefits that accompany smiling and having fun, including lower stress levels, which is good for your heart, and better brain function as you age. If the play is frustrating or stressful, you won't get the heart healthy benefits of playing games. It might still help you keep your brain active, but if your heart is not healthy, your brain suffers over time.
When it comes to an educational game, there are many factors to consider. If the software doesn't measure up, it is simply a waste of time to use it.
Many of the free educational games available online are designed for vocabulary building. A vocabulary game can be fun for hours when it is designed to be more fun than concentrating solely on the educational purposes. Even Scrabble is helpful for building vocabulary, although that's not why most people play the game because it is very fun.
Crossword puzzles can be used in all subjects, including spelling, health, science and even more advanced subjects like chemistry. The puzzles combine vocabulary with definition of terms that make it fun to figure out. Foreign language crossword puzzles are also available.
Some online programs are basically electronic flashcards. One side of the card might be a math problem, a definition, a foreign language phrase or a chemistry problem. The other side will reveal the answer. You will also find many trivia sites that will rank you against people all around the world which can make it fun and not to mention make a few friends along the way.
The object is to look at the questions and the answers as they flash by, for a certain period of time. The second step is to try to remember the answers and say the answer before the card flips over. Sometimes the cards flip automatically. Sometimes a mouse click is used to flip the card when you are ready.
You might think of free educational games as "serious". That is how they are sometimes referred to. But that doesn't mean the game cannot be fun. It should be fun. If you are looking for something for a child like abc kids games, it must be fun or it will not hold the child's attention.