  Aims. A sample of early B-type stars in OB associations and the field within
the solar neighbourhood is studied comprehensively. Present-day abundances for
the astrophysically most interesting chemical elements are derived. Methods.
High-resolution and high-S/N spectra of early B-type stars are analysed in
NLTE. Atmospheric parameters are derived from the simultaneous establishment of
independent indicators, from multiple ionization equilibria and the hydrogen
Balmer lines. Results. Teff is constrained to 1-2% and logg to less than 15%
uncertainty. Absolute values for metal abundances are determined to better than
25% uncertainty. The synthetic spectra match the observations reliably over
almost the entire visual spectral range. Conclusions. A present-day cosmic
abundance standard is established. Our results i) resolve the discrepancy
between a chemical homogeneous local gas-phase ISM and a chemically
inhomogeneous young stellar component, ii) facilitate the amount of heavy
elements locked up in the interstellar dust to be constrained precisely:
carbonaceous dust is largely destroyed inside the Orion HII region, unlike the
silicates, and that graphite is only a minority species in interstellar dust -,
iii) show that the mixing of CNO-burning products in the course of massive star
evolution follows tightly the predicted nuclear path, iv) provide reliable
present-day reference points for anchoring Galactic chemical evolution models
to observation, and v) imply that the Sun has migrated outwards from the inner
Galactic disk over its lifetime from a birthplace at a distance around 5-6 kpc
from the Galactic Centre; a cancellation of the effects of Galactic chemical
evolution and abundance gradients leads to the similarity of solar and
present-day cosmic abundances in the solar neighbourhood, with a telltaling
signature of the Sun's origin left in the C/O ratio. (ABRIDGED)
