Did you know that the original meaning of the word "weekend" referred to the time from noon Saturday to Monday morning? Did you also know the word "download" seems to have originated in 1980? AND, did you know I spent a little too much time reading about the etymology of these words?
Meritous (Windows, 3MB, free) - A very simple but wholly addictive exploration-based role playing game with strong action/arcade leanings. Shuffle through rooms in the dungeon dispatching enemies with your chargeable psychic powers. When they die, grab the crystals and use them to upgrade your stats. And of course there are treasures to be found and bosses to be fought. The color of the rooms indicates how much danger you're in, and depending on the type of music humming in the background you can tell what part of the map you're in. Highly recommended.
Marathon Trilogy (Mac/Windows/Linux, various, free) - The spiritual predecessor of the Halo series, Marathon: Aleph One, Marathon: Durandal, and Marathon: Infinity are first person shooters created by Bungie and originally released in the mid 90s. Now all three games are available free as open source projects. You take the role of a security officer aboard an enormous starship in the year 2794. Defend the crew from a race of alien slavers and slowly uncover the corruption amongst the humans you once trusted. Don't expect top-notch visuals or modern play control, but do expect a story-driven experience and a surprisingly deep atmosphere. Note: To play the games, download an episode along with Aleph One for your OS. When both are unpacked, copy Aleph One into the appropriate Marathon folder and you're good to go.
Verge (Windows, 7MB, free) - A 2D platformer created by the developer of Bonesaw for the TIGSource Commonplace Book competition. Take control over the unnamed protagonist who must travel between two worlds in order to solve simple puzzles. Impassable blocks in your way? All you have to do is die — that's right, go step on some spikes, Karoshi style — and you'll be transported to the flipped underworld where you'll likely find a way to get through. But be careful, as enemies in this dark realm will attach to you and steal your energy, so make your trip through the underworld a short one.
Cosmic Invaders (updated version) (Windows, <1MB, free) - If Space Invaders were a tower defense game, it would be Cosmic Invaders. Buy and deploy one of several types of turrets at the bottom of the screen. As the aliens descend from above, they fire and destroy as many of the buggers as they can. Spend the cash you earn to repair and upgrade turrets whenever you have a spare second.