Queens of the Stone Age: opening for the Smashing Pumpkins at Trammps in New York City, a few months before the Pumpkins called it quits. They'd just released their first EP. They were awesome, still had a very Kyuss-y sound to them (which I found out much later when I actually heard about Kyuss)
The Used: I heard Maybe Memories on an Ozzfest sampler and loved it, and a couple months later, they started playing The Taste of Ink everywhere, and Kelly Osbourne hooked up with Bert McCracken... The Used are really on a roll with releasing all their worst songs as singles now
Chevelle: Heard them play at the same Ozzfest, they kicked ass, I picked up their older CD, and not too long after, The Red just exploded.
Lostprophets: OK, so they aren't a huge band, but their next album (coming out in the fall) could quite possibly lead them to greatness. If you read their website, you can see why.... they're trying to make a "real album" now, with heavy production and whatnot. I can't wait to hear the actual material. I started listening to them when I caught a sample of their music on a Virgin Airlines flight to London in Feburary 2002... not too long after, the Shinobi Vs. Dragon Ninja video got some airplay on MTV2, and they appeared on Ozzfest that summer.