I can't rave about my Shure 535 IEMs enough. I'm even about to spend the $2-300 to get custom molded fittings for them Near-reference quality sound (a little too warm, though it hasn't been an issue so far), great isolation (about 25db), and they are hard to have randomly pulled out of your ears.

Granted, this is well out of the $100 ballpark, but they're still fantastic.

If you're looking for reference quality sound at under $100, look into the Etymotic in-ear headphones. Ridiculously good, though they are a bit weak on the low end of the spectrum (not enough that you won't feel bass on real punchy songs, but enough that they don't sound like any other headphone on the market).