Norah Jones.
Anime.
I saw Billy Joel in concert right before he released “Cold Spring Harbor.” I knew he could be big, but I never imaged him being huge.
My sister-in-law was at one of the first Garth Brooks’s concerts, and just fell in love with him.
Going out on a limb here, But…
a.) Didn’t their first album just come out 8 months ago?
b.) are they popular now?
I caught Blink 182 at a show YEARS before they hit big, and were still playing some covers along with their regular set. They rocked my world and I’ve been a fan ever since, even though a lot of their music now is geared towards a teenybopper audience.
FWIW, I also premiered Korn’s “Blind” at a Midwest college station. The CD had been provided by the record company as a “new emerging act” and wasn’t even professionally pressed - it was on a blank silver CD, no words, no cover, no nothing. I thought it was pretty good.
Liking the Postal Service before it was cool just seems impossible to me, as the entire project was cool before the album hit the shelves. Gibbard and Tamborello’s involvement with DCFC and dntel was already cool, sorry. 
This is an odd one but…
in 1987 I decorated my entire house in retro 1950’s furniture and appliances. It was an obsession! I had so much fun when people came to visit me and I saw the weird looks on their faces. I got tired of it and gave away most of that stuff and now I could probubly get thousand for what I had.
I also took great pains to amass a huge collection of lounge music albums for the complete bachelor-pad effect. Now all those mondo-lounge albums compilations come out on CD and I say, “Hey. I’ve got the record!”
Ditto on the R.H. Picture Show. First played Riff-Raff in Kent Ohio in 1977 or 1978.
Is collecting sea-chanties cool yet?
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Single Malt Scotch - spent my junior year of university in Scotland in the early '80’s, learned to drink there (they take it pretty seriously there, if you haven’t heard), came back and by the late '80’s Single Malts were the Cool Thing. It was a pain, because everybody in the U.S. was so proud of loving Glenlivet or Glenfiddich and didn’t get that those were okay at best. After a few years, though, I didn’t mind the trendiness - at least it led to the good stuff becoming more available over here…
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The X-Men - as a 12 year old, I bought issue #95 at the newstand (death of Thunderbird), loved it, went back and bought #94 (first regular issue of the new X-Men), Giant Size X-Men #1 (the modern equivalent of Action #1, Detective #37 or Amazing Fantasy #15 - and I paid $5). I bought the regular issues for 2 - 3 years, and traded a bunch of other comics to get earlier X-Men issues from the first gen of the title - I had the Steranko Sentinel issues, a bunch of stuff back to #9 - about 60 of the first 93…then, like a complete flippin’ bonehead, I sold them for a cheap-ass guitar amplifier when I was 18. I got $60 for Giant Size (now worth at least $3,000), and 4-5 for issues #94 - 120 (issues #94 - 105 are each worth several hundred at least). What a schmoe.
That’s a start…
The Onion - I used to pick it up on the UW campus when it was a B&W weekly and had about 12 pages in it.
Nirvana - Saw them in Minneapolis is Spring (April maybe) of '90. Wish I still had that ticket stub.
I was a Titanic nut years before the movie came out and suddenly everyone became experts. And now that the subject has been done to death in the public forum, my interest in the matter has…well…disintigrated.
I decided years ago that a Jeep Grand Cherokee would be my dream vehicle. Then SUVs became the “in” thing. Now I drive a station wagon, thanks very much.
jk1245I used to pick up the Onion when it was four pages :), and then really only for the coupons (it took them a while to really find their voice).
I was into cream soda before it was cool, oh wait, it still isn’t.
MMORPGs
I was playing MUDs and MOOs over a shell account on a Mac LCII in 92 or 93, long before any of this everquest and FFXII garbage.
Lord of the Rings.
My father read me the books when I was a kid, and I’ve re-read them many times since. Now it’s the hottest thing around, because of the movies, but reading those books at school 10 years ago got me labeled a geek.
Grunge.
I bought Pearl Jam’s “Ten” when it first came out because Stone and Jeff had been in Mother Love Bone.
Also Tool, Blind Melon Rage Against the Machine. (I met Zack at the old 9:30 club when they were opening for Shudder to Think and the guitarist in my old band got himself back stage at early shows by Tool and B.M. because we were fans of those guys when they were just starting out and far from their home crowds on first/early U.S. tours.
YES! Iron Chef on KTSF! The subtitles were sooo much more entertaining than the dubbing added for when IC went to FoodTV. I always wished I’d taped those episodes.
Tori Amos (as a solo artist). I heard her on CBC radio on a show called Brand X, in her first exposure in Canada, and was transfixed. I wrote down the title of her album, and as soon as we piled out of the van in Barrie, I bought it. It floored me.
I got my best friend, Meghan, to listen to it. At first, she was like, “What is this crap?”, but it grew on her and soon every time she bought tickets to a “Tee-show” she’d get an extra one for me. In high school, three years later, a boy transferred to my class and I leant him my Tori Amos tapes. He became one of the biggest EWFs who ever lived, and started telling everyone that he had turned me onto Tori. I just smirked and produced a note he’d given me back then that said; “Thanks for the Tori Amos tapes, they were great.”
Bonsai, single malts and maybe home brewing (at least around here).
Ok, maybe I’m still leading the pack with the bonsai bit but I’m telling you it’s coming! Tiny trees! Tiny pots!! 
Moby.
About 15 years ago, he used to write MODs of his music and distribute them over the Usenet. I collected a bunch of his stuff for FREE.
I read the Straight Dope for years and years before this site was a twinkle in Cecil’s modem.
Free-style mogul skiing. Was approached to try to promote it as an exhibition event for the 84 winter Olympics. Mom and Dad put the Kibosh on that right quick.
She told me she loved me like a brother. She was from Arkansas, hence the Joy!
I was into “the Simpsons” before they were cool. I remember watching the original shorts on the Tracy Ullman show. I even remember thinking they were the funniest bits on the show.
When Fox broadcast the original season of episodes, I had to argue with my brothers to not turn the channel when it came on. “It’s a fuckin’ cartoon!” my brother yelled. Of course, they laughed themselves silly. I even remember the episode - the one where they attend the company picnic at Mr. Burns’s estate, and the family embarrasses Homer.
I also had to argue with my brother to go with me to a free concert at University of Buffalo by this new band I had heard of - the Red Hot Chili Peppers. Again, he resisted my suggestion, but was instantly turned into a Chili Pepper fan from then on. A month later, the local rock station played “Higher Ground” for the first time on the radio while we were driving home from work. My brother blasted the tune.
My dad managed to persuade a certain indie band who had fairly recently signed to his record company to let his son’s band borrow their monitor speakers for a gig, as they wouldn’t be needing them for a while, as they were going into the studio to record their first full album.
I picked up a copy of the Drill EP to see who it was who was being so generous and loved every note. Ditto for Pablo Honey and The Bends. Then they started getting weird and full of themselves and I went off them around the time everyone outside of the UK discovered them.