  We exploit the spectral archive of the Sloan Digital Sky Survey (SDSS) Data
Release 7 to select unusual quasar spectra. The selection method is based on a
combination of the power of self-organising maps and the visual inspection of a
huge number of spectra. Self-organising maps were applied to nearly 10^5
spectra classified as quasars by the SDSS pipeline. Particular attention was
paid to minimise possible contamination by rare peculiar stellar spectral
types. We present a catalogue of 1005 quasars with unusual spectra. This large
sample provides a useful resource for both studying properties and relations
of/between different types of unusual quasars and selecting particularly
interesting objects. The spectra are grouped into six types. All these types
turn out to be on average more luminous than comparison samples of normal
quasars after a statistical correction is made for intrinsic reddening. Both
the unusual broad absorption line (BAL) quasars and the strong iron emitters
have significantly lower radio luminosities than normal quasars. We also
confirm that strong BALs avoid the most radio-luminous quasars. Finally, we
create a sample of quasars similar to the two "mysterious" objects discovered
by Hall et al. (2002) and briefly discuss the quasar properties and possible
explanations of their highly peculiar spectra. (Abstract modified to match the
arXiv format)
