Cave Story is an excellent Metroid-esque action adventure game with a bizarre story and a metric ton of levels to explore. The player takes control of the main character navigating the labyrinthine cave,  collecting weapons and power ups to access later stages (much like Samus). The title does have a PSP version, and a DS incarnation is in development, but the original is still free for the taking.

2. Marathon Trilogy

Halo’s famous predecessors, the games of the Marathon Trilogy offered evolved, excellent FPS gameplay when the genre was in its bloody infancy. Featuring an awesome sci-fi narrative told entirely through the gameplay and plenty of satisfying alien gunplay, it’s easy to see Halo’s roots. An enhanced version of Marathon: Durandal (the 2nd game in the series) will soon be available on XBLA, but all three titles still free and just as awesome 13 years later on Bungie’s site.

1. No Action Jackson

Recalling classic LucasArts adventure titles (down to the interface and cartoon-y graphics) No Action Jackson is a polished, hilarious take on the genre. It features the same kind of humor – centering on a kid who dreams of becoming a dungeon master- and puzzle-centric gameplay of the early 90’s adventure heyday.

Honorable Mentions

Johnny RPG 

Every Extend 

Future Pinball

Harmotion

Sauerbraten