A great fan made Halo spinoff that ties in well with the actual storyline.
The gameplay actually has a pretty wide variety of stuff. It has almost all of the weapons from the original Halo (along with the Halo HUD) and all are very accurately portrayed. To add a few breaks in the action there are scenes where you drive a warthog and shoot enemies as the gunner. As a short special treat, you use a turret on the back of a pelican at the end of the game.
Halo Zero is not like other Halo games, mainly because it is a 2D sidescroller. This means the graphical quality isn't quite as high (that and it being a fan made game) but it doesn't mar the experience. Most of the joy you get from Halo 1 and 2 (not to mention almost the entire soundtrack) is alive and well in Halo Zero. Just don't expect it to last too long. Once you know how to do everything the game can be beaten in around 30 minutes, and even after that it's still a great time-killer.
Any fan of Halo 1, 2, or 2D sidescrollers should get this game. After all, it's free.