Tropers/45xxx TV Tropes has quickly become the website I visit the most (after it was mentioned on Atop the Fourth Wall)... however, I use the internet so much that I consider it a replacement to television, you can take that statement for what it's worth.
My Little Pony: Friendship Is Magic: After reading the pages for the twopart pilot at around 3AM in August (2011, at time of writing), I decided to look up the episodes on YouTube. And now I'm a brony. In fact, thanks to discovering their pages here, I also really love the following fan fics:
Ack Sed This is what the wiki is for - feeding my Archive Binge tendencies. I seem to seek out the little gems that no-one has heard of, and that I have a devil of a time convincing people to read/watch.
Avatar: The Last Airbender: Natch. I wasn't disappointed, either. I should say that it isn't anime - there's far too many Western animation elements for that. This also introduced me to shipping and the cracky goodness of CAPSLOCK livejournal communities.
Empowered: I really do read it for the story. My experience went like this: "Ooh sexy woman with clothing damage, must read." - Later - "Okay, fanservice that reduces the subject to a quivering wreck is not funny." - Later - "Thugboy/Emp OTP for evah, yo!"
Ciaphas Cain: Read its quote/entry on Dojikko, then the entry itself, then went straight out and bought the Hero of the Imperium collection. I'm becoming interested in tabletop Warhammer 40,000 and the other novels as a side effect — the Rule of Cool setting pleases me.
Mysterious Girlfriend X: Read the entry, thought the concept was weird, unique and cool, checked it out and started reading.
Mystery Science Theater 3000: I'd never watched this before apart from the movie, but now... well, all the quotes made me check it out, though I'm not masochistic enough to sit through really bad movies, even if they generated the best snarking. One of these days I will actually sit through a full episode rather than hunting out "Best-of" clips.
Dollhouse: Watched up to the third episode because it was a Whedon series, then dropped it because it didn't engage me. Gave it one more try when it started popping up in other entries, and pushed through to the other side of ep. 6.
Kimi ni Todoke: The forums were gushing about it, I checked it out, fell for it, hard, and became that thread's most enthusiastic poster. Produced the most emotional reaction I've had to an anime ever. A refreshingly straight love story — no hilarious misunderstandings, no contrived stupidity and it makes sense.
B Gata H Kei: First episode is a living nightmare of Cliché Storm hell, but you soon pass through to the sweet sex comedy centre. Another forum find.
Bunny Drop: Wondering at the odd concept in the entry (someone raising his 6-year-old aunt?), I began reading. What I got was an honest lesson in becoming an adoptive parent. It's the yang to Yotsuba's yin, especially as it averts Not Allowed to Grow Up.
Nana to Kaoru: Three people namechecked this, so what else could I do? I can honestly say it's been educational.
Animaniacs/Pinky and the Brain: With all the quotes, I had to check them out — I wasn't disappointed. It was very very very very very very very very very very *bonk* funny. My 8-year-old nephew loved it, though he liked PATB more.
The Ren & Stimpy Show: And this was the one he liked most of all. Seriously, he wet himself laughing at Powdered Toast Man. I nearly joined him.
Himawari!: One offhand link in the forum led me to this. I would have never even imagined this existed, but here it is. Imagine a Lighter and SofterNaruto but with all the cool ninja tricks intact. (Later) Ecch, only watch this if you have "Easily Amused" tattooed on your forehead. Looks nice, shows promise but NOTHING HAPPENS.
Planetes: In the "Hard SF" thread, the one series that kept popping up was this one. ONE episode - that's all it took for me to fall in fan-love.
Gunnerkrigg Court: I tried - really tried - not to add more webcomics to my daily list. This little slow-burner made me break that rule. Charming, eerie, expressive Urban Fantasy about a strange girl in a stranger school.
Blip: Needs More Love. What looks like "Breezily sarcastic slacker hangs out with her friends" reveals itself to be something more. You know how sometimes you have the feeling that Fate is holding you back? For her - it is.
Erfworld: Don't quite remember where, but one of the articles had a link. Tabletop RPG gamer gets transported to a 'game' world (or is it?). He soon discovers that it's not that much fun when you see people die in front of you. They discover that Humans Are Cthulhu.
Quite a lot of Western Animation that I might not have watched otherwise. It introduced me to the very idea of a Periphery Demographic: it might not be aimed at me, but if it's enjoyable enough, who cares?
Avatar: The Last Airbender: I had seen some isolated season one episodes before, but after seeing it mentioned again and again on this site, I decided that it might be more than just another kids' show and started watching it from the start. Boy am I glad I did.
The Order of the Stick: I'd never even heard of it before. Now not only am I addicted, my little brother heard me mention it and decided to have a look. He's a fan now too.
That Guy with the Glasses: Okay, I watched Ferngully as a child. I watched it a lot. Someone linked to The Nostalgia Critic and The Nostalgia Chick's review of it . . . and I was hooked to his reviews at least. I probably would have just spent my time watching reviews by the Nostalgia Critic and the Nostalgia Chick reviews if it wasn't for people on this wiki mentioning Linkara . . . so I watched one episode of Atop the Fourth Wall, and was hooked. And given the number of crossovers with Spoony and the awesomeness that is Doctor Insano, I was soon watching The Spoony Experiment as well. I now regularly watch the videos of several other Channel Awesome members as well.
xkcd: I'd heard of it before, but it was TV Tropes that got me interested.
Thursday Next: Okay, Jasper Fforde is now one of my favorite authors. I'd never even heard of him before.
Yu-Gi-Oh! The Abridged Series: Why on earth would I want to watch a Gag Dub of a children's show that I've never watched about a card game I've never played and have no interest in? Gosh, a lot of people on TV Tropes seem to like it, and the first episode is only around four minutes, I might as well see what all the fuss is about . . . and the rest is history as they say.
Chuggaaconroy- I was curious as to what an LP was(even though I've read a screenshot LP of Pokémon Mystery Dungeon before, but I didn't know it was an LP). I found Chuggaaconroy and started watching his videos.
The Dresden Files. What really got me was the example on the Dangerously Genre Savvy page with Nicodemus and Harry. I'd seen it listed all over the site before that, but that was what actually made me seek out the books.
Kings. Unfortunately too good to last, but it's on Hulu.
Reaper. Another one cut in its prime, but also on Hulu.
Gotham Central likewise. In retrospect, I actually started reading this before 52 because I knew it was part of the backstory, and it came highly recommended, but while this got me back into comics, I'd say it was 52 that got me back into superheroes.
Dragon Age: Origins. I was considering buying it, but it wasn't until I read about it on the site that I got really excited for its Australian release.
Firefly: Never would've known it had ever existed if not for this site. On June 28, 2013, at 6:26 PM Winnipeg time, the Netflix connection with the series finale, "Objects in Space," terminated in my basement.
Arrested Development: I knew it was there and had tried it out once, but picking up in the middle of Season 2 was horribly unhelpful to me, so I gave it up. Then, just a couple of months later, people in my social circle commenced the name-dropping of it. Now that my family has Netflix, I have started watching it, and is it ever good! (Of course, the first three seasons have something of an Unintentional Period Piece aspect to them, but I don't complain about that.)
Community: Like Arrested Development, it was popular at my high school. Like Firefly, I never knew it had existed.
Heck, this could apply to all the scripted live-action TV I've watched recently.
Some of these examples here are only semi-gateway drugs, while some others are ones I've completely met through TV Tropes. For the semi-gateway drugs, I've known about some of them, before, but TV Tropes made me completely establish them as a part of my life.