Has anyone besides me ever played Chex Quest? It is so fun! You play as a Chex Warrior and use your zorchers to send Flemoids(large walking goo balls) back to their dimension. There are 3 original games(free) and also some fan-made versions(also free and the most notable being The New Chex Quest: Generation 2), and links to download these games are found on the Chex Quest Wiki at chexquest.wikia.com and you can find the download for all three games in one .zip Here.
Also, If you like rogue-like dungeon games(y'know, fighting monsters and stuff like that), try Moraff's World. This game isn't entirely free, but you can play the game for free if you don't mind being a bit into the game and not getting to create your character. Either way, there are 2 sequels to the game: Moraff's Revenge and Moraff's Dungeon of the Unforgiven, and you can purchase all three online for only $20.
Finally, a game I know some of you have played: MYST!!! This game is mind-blowingly hard, but so much fun. My two of my sisters and my dad have played this game and gone on to play the next several in the series. There are 6 in this series: Myst, Riven, Exile, Uru(a step out of the line of the games), Revelation and Myst 5(real creative, right?). These are not free but they are extremely fun if you like a challenge.