What kind of bike are we talking about here? XC, freeride, DH??
For XC bikes I would say anything ~26lbs or less would be considered lightweight.
Tires/wheels make the biggest difference, and offer the most bang/buck in terms of weight loss. Some OEM forks weigh a TON, and so do drivetrain components, but those cost a lot more to upgrade.
As for scales, I picked up an Ultimate Alpine scale from my LBS. It's about $60 and clamps on to your workstand. It's great, and very accurate!