D&D: 1979, AD&D 1st edition. My two cousins introduced me to the game, and despite having to choose the least-sucky PCs from a group of about a dozen that they had used a computer to randomly generate six ability scores from 3-18 (on a flat curve, instead of 3d6) and nearly getting killed by giant rats, my two brothers and I were instantly hooked. We got the three core books as presents that Christmas from my mom.
Star Wars: 1977, saw the first movie in the theater the year it came out. Saw the rest of them that way, too, although I never bothered with Solo or the last one of the "core 9." Maybe I'll get around to them someday, but I'm in no real hurry - I'm really not all that much of a Star Wars fan.
Earthsea: I was aware of their existence when I joined the Science Fiction Book Club in high school (for the first time of many; I'd join to get four books for a dollar, buy the requisite four more at full price, quit, wait a bit, then join again to get four more books for a dollar, and so on), but I never read them. I've read a few of LeGuin's short stories, but that's about it.
Harry Potter: My son read them when they first came out and talked them up enough to arouse my interest, so I read his copy of each book as it came out, once he was done with it. (Years later, I'd return the favor with the Gentlemen Bastards series.)
Magic The Gathering: Never played (no interest), probably first became aware of the game through Dragon magazine ads.
Marvel/DC: Hoo boy. I've been aware of the main superheroes in each franchise via cartoons via Saturday morning cartoons (Super Friends, the original Spider-Man cartoon, etc.) from when I was little, but my first comic book purchase wasn't even a superhero comic. I was in a convenience store one day when I was a sophomore in high school (that would be around 1979-1980 or so) when I spotted a John Carter: Warlord of Mars comic on the spin-racks. Having been a big fan of the Barsoomian novels, I picked it up immediately, and then noticed they also had a comic book of Godzilla, so I had to buy that as well (although the Marvel Godzilla wasn't drawn as accurately as I would have preferred). After a few months of checking out the spin-racks for my two titles, I saw Amazing Spider-Man issue #196 (or thereabouts), with the blurb "The Final Fate of Aunt May!" blazing across the top, and I thought to myself, "Wait - they're going to kill off Peter Parker's Aunt May? This I have to see!" since I recalled her from the cartoons I watched as a kid. It took a few issues later to find out that was a hoax (she wasn't dead after all - whew!), but I was now up to three Marvel comic books a month. And then Spider-Man had a crossover storyline with the Fantastic Four, and I started picking up that title as well...which eventually led me to getting into the Uncanny X-Men, the Avengers, Iron Man, Daredevil, Doctor Strange, and so on. Without quite understanding how it had happened at the time, I had fallen fully into the Marvel Universe. I've been a comic book collector ever since, although I've dropped back significantly on how many titles I read (comics were 35 cents when I started collecting; now they're routinely $3.99). And I never got that much into DC other than Batman solo titles (mostly Legends of the Dark Knight and the occasional Elseworlds one-shots, as I enjoyed the character of Batman but really couldn't tolerate Robin), and then later Saga of the Swamp Thing (when Alan Moore was writing it) and John Constantine: Hellblazer; I picked up the complete Sandman collection in graphic novel format after the fact.
And while there are a bunch of other topics having been added on since, I'll just mention one particular one (this is already a long post!), since it's had such a big impact on my life:
Doctor Who: I first saw a few of the Jon Pertwee (Third Doctor) serials as Saturday night specials on our local PBS station during Pledge Week. I enjoyed them so much that as a freshman in high school, one week after joining the track team (I was a fast sprinter in middle school, and the coach found that out and pressured me to join the team), I immediately quit when the PBS station started airing the Pertwee episodes every weeknight at 5:30 PM. I couldn't do track practice and watch Doctor Who, so track was right out. (I remember my mom was kind of pissed at first, since we'd had to pay $20.00 for my sports physical, but she ended up being a Doctor Who fan as well.)
And, as odd as it sounds, I owe my marriage to Doctor Who. In the summer of 1983, after my freshman year of college, I was back home and I volunteered to help out at my little sister's middle school library, aiding with their book inventory. That's when I found out the head librarian, Mrs. Adams (who was divorced with two daughters she was raising alone) had - just like me - tickets to a Doctor Who convention in Chicago the following weekend. We decided to save money and carpool to the convention. And, since this was 1983, the 20th anniversary of the show, there were a lot of Doctor Who conventions in Chicago that summer - we hit four of them all in all. And somehow, by the end of that summer, "Mrs. Adams" stopped being my Doctor Who convention carpool partner and had somehow become "Mary," the woman I couldn't imagine not spending the rest of my life with. We got married the following year (I was just shy of 20, she was 35), and next month we'll be celebrating our 39th wedding anniversary. And all because of a TV show about a Time Lord in a little blue box, traveling the Universe and helping people.
Johnathan