I know people don't spend a lot of time reading these reviews so if you read nothing else, read this: Treasure Adventure Game isn't just one of the best free games available today, it's one of the best games available period.
"Metroidvania" games, as they're known colloquially, are a dime a dozen these days. Even freebies like Cave Story are readily available. What Treasure Adventure Game does differently is truly allows you free reign. After about an hour into the game, you can go anywhere across the entire world. While it's not the largest game world out there, it's still quite large and is packed with exciting opportunities.
The guided section in the first hour or so is just enough to give you a wonderful introduction to the game and allows you to see several of the areas that you will be able to reach later on in the game. There are plenty of items to collect throughout the game world and each allows you significant advances in where the player can go. For example, Boots allows the character to jump higher and therefore access new areas. Some aren't as cliche as that, however, such as adding a cannon to the boat which lets you shoot down switches and fight enemies on the high seas.
Aside from searching for power-ups, your goal in the game is find 12 treasures scattered across the world. These would be nearly impossible to discover if not for maps similarly lost. The maps are usually in fairly obvious locations although they aren't all easy to get to. The maps then lead to secret locations for each of the 12 treasures. There were times I felt aimless but they were short and usually accompanied with an "ah-ha" moment that felt satisfying and rewarding.
The dungeons are often large but manageable. They never felt so huge that I felt lost but they also weren't over too quickly either. The bosses are also appropriately epic and a lot of fun to fight. Not all of them are classic but all are fun and well worth the playthrough.
The plot isn't the greatest you'll ever experience but for a free, indie game, it will probably surprise you. The story has twists and turns, some you'll see coming and some you may not. Considering how great the rest of the game is, the ending is a bit of a letdown with a bizarre and out-of-place final battle with a rather anti-climactic ending that seems to lose faith in the quality of the game. Still, by this point, you're probably already feeling like the game was well worth the price of entry and some may find the ending amusing or even exciting.
Overall, Treasure Adventure Game is a wonderful game, made only better by the fact that it's free. Do yourself a favor and don't assume that just because it's free means it's not worth any cost.