One of the first computer games I ever played with any real understanding was Breach for the early Macintosh, circa 1985. It was a 2-D turnbased action game, where you had a number of, yep, space marines, to attack a base or destroy different kinds of aliens, stationary guns... I don't actually remember too many details about it and I couldn't find anything at all on the interwebs about it; it's just too long dead. I think there was even a simple level editor with which you could create missions for yourself or others to work through. There were a couple of sequels but I don't think they ever did too well - I read that Breach 3 was pretty sloppy. It is strange to me that there's so little information online about the early macintosh games - Airborne, Captain Magneto, Enchanted Scepters (the World Builder showpiece)... In any case, it set the stage for my future love of all things space marines.
I became obsessed with Aliens sometime in early high school. The raw fear and suspense of the first movie was lost on me (I appreciate it more now), but the snappy dialogue, futuristic technology, and combat action of the sequel was apparently just what my doctor ordered. Not to mention the xenomorphs were given much more screen time - throw in the final confrontation with -gasp- an enormous queen, and it was a slam dunk. No matter what other roles they might inhabit from that point on, Michael Biehn would forever be Hicks, and Bill Paxton... "Hudson, sir. He's Hicks." The great appeal that the plot points and imagery of Aliens had for me would later transfer directly to the Tyranids. Such as it is, even a 'nid player can appreciate the smug satisfaction of an opponent as they blast apart your bugs; you know you're the bad guy, and anyways, you can take comfort in remembering that (usually) the tide will turn when you reach his lines and your depleted forces still tear his troopers limb from limb.
I didn't see Patton until years after I first started playing 40k, but it stoked my fire for playing Imperial Guard like no other war movie had done prior. After watching Patton's armor, infantry, and artillery reduce Rommel's column to chaos in a spectacular desert ambush, it set the wheels of my imagination working. This geargrinding would persist and finally culminate with the completion of my own guard army - the 14th Tiefwalder. Obviously, Platoon and Full Metal Jacket lent their own imagery. I always really liked the scene late in Jacket when a single tank arrives to assist the pinned-down soldiers, providing cover for the infantry and destroying the sniper-infested buildings one at a time.
Anytime I need extra inspiration to paint or play some Space Wolves, I turn to The 13th Warrior. Sure, there's some cheesy shenanigans (mostly involving Banderas), but the pitched battles between the enormous, fur-clad warriors from the north and the insane, primitive bear folk are great stuff to get the norseblood flowing. For the same reason, I was disappointed to hear bad things about Pathfinder. Oh well.