For me it would be an Australian band called Something For Kate in 1999. I had been listening to Triple J (nationwide 'alternative/indie' govt-funded radio station that was a revelation to a kid in a country town with only top 40 or commercial country) for a few months, but they were the first band I became obsessed with - I actually ended up buying all 4 singles from their album that year without deliberately meaning to. Pretty much a prelude to wanting to own lots of music and search it out.

As well, Radiohead - I remember buying OK Computer later in 1999. The idea of rock-type song having different movements etc

Joy Division opened up the idea that 'old' music could be just as awesome (or more so) than current stuff

Oh, and as much as it gets criticised, stumbling upon Pitchfork one day made a huge difference in exposing me to music that was only just bubbling up overseas, and that hadn't made it to Australia yet. I clearly remember reading the top 100 for whatever year Turn out the Bright Lights came out and being intrigued, then loving it when it started getting played on Triple J a few months later

Same here - but also for the under-the-surface sensitivitiy and the right doses of clever songcraft.

Also, they dressed cool, had cool hair, and were midwestern. They drunk beer, had short albums, and understood the band-as-brothers mythos (before they fired a brother). And they had some great sad songs and lines.

well, it would honestly be hootie and the blowfish...

if alternative radio never started playing them so fucking much causing me to switch to the college station to hear new music, I wouldn't be where I'm at today.

I so remember singing "rock lobster" in middle school gym class with a couple of other geeks that made up my clique..those were the daze.

public enemy and tiger trap

Radiohead definitely had more of an impact on what I tend to listen to the most now. Even when I was listening to alot of early punk and heavily into the early to mid 90's Dischord bands, I listened to The Bends alot.
I think NMH, Beck and Radiohead really mellowed me out in the late 90's. Withouth listening to Radiohead, I would have never checked out Sigur Ros, Four Tet, Clinic or Can.

Lithpt, much?... Maybe you just don't know how to spell.

Meat Puppets...
Without them I might still be mired in a sea of "Classic Rock" and metal.

Metallica
Nine Inch Nails
Jane's Addiction
Stanley Clarke
KoRn
DJ Shadow
Built to Spill
Sunny Day Real Estate
Sigur Ros

I honestly don't know. Maybe it was Nick Cave. He led me to Neubauten, Die Haut, The Gun Club, Anita Lane, Dirty Three, PJ Harvey, Cat Power (through D3), Tindersticks.

Yeah it was Nick Cave.