Half-Life. Hail to the classics!

I don't know how so many people haven't played the original Half-Life.  To me, it is one of those standout games that everyone should play.  It doesn't matter that the game is 16 years old, it is still extremely playable (or replayable).

Maybe you're one of the few unlucky people who haven't had the chance to play Half-Life, in that case I feel bad for you. Half-Life is a first person shooter that follows Dr. Gordon Freeman a theoretical physicist at a secret lab called Black Mesa.  When a secret experiment goes awry an interdimensional rift is opened allowing aliens from another world to invade.  Gordon must fight his way through the lab, facing lots of strange aliens (and humans) and often solve different puzzles/obstacles in order to advance.  The weapons that are available throughout the game range from the conventional pistols, machine guns to the unconventional--a crowbar?  If the zombie apocalypse ever happens, remind me to buy a crowbar.  Gordon also has access to his haz-mat suit which provides some shielding and can be recharged.

Shootin' at a helicopter--I'm sure that'll work.

Half-Life was followed up with two additional content packs--Blue Shift which followed a security guard at Black Mesa and takes place parallel to Gordon Freeman's story.  Opposing Force focused on the soldiers who were sent in to contain the alien threat.  They're not quite as fun as the original game, but are enjoyable side adventures.

Half-Life was really the game that lauched Valve and spawned the 2004 follow-up Half-Life 2 as well as the unfinished add-ons in episode 1 and episode 2.  Whether or not Valve ever decides to finish the series remains to be seen, but you can't go wrong starting with the original.