A rapid screening and enrichment method is reported for exploring the size and shape diversities (libraries) of nanometer-scale colloidal particles. With the use of surface-enhanced Raman scattering, a new class of metal nanoparticles has been identified from heterogeneous silver colloids. These particles are relatively large, faceted nanocrystals that are able to enhance the efficiencies of surface optical processes by as much as 14-15 orders of magnitude. The population of these novel nanoparticles is only about 0.1-1% in standard colloid preparations but can be enriched to about 10-15% by size-selective fractionation. This new class of nanoparticles could find potential uses in optoelectronic devices, ultrasensitive chemical sensors, and single-molecule detection.