We have carried out a search for Wolf-Rayet galaxies in all galaxies with EW(Hb) > 2Å in the SDSS DR6. We identify Wolf-Rayet features using a mixture of automatic and visual classification and find a total of 570 galaxies with significant Wolf-Rayet (WR) features and a further 1115 potential candidates, several times more than even the largest heterogeneously assembled catalogues. We discuss in detail the properties of galaxies showing Wolf-Rayet features with a focus on their empirical properties. We are able to accurately quantify the incidence of Wolf-Rayet galaxies with redshift and show that the likelihood of otherwise similar galaxies showing Wolf-Rayet features increases with increasing metallicity, but that WR features are found in galaxies of a wide range in morphology. The large sample allows us to show explicitly that there are systematic differences in the metal abundances of WR and non-WR galaxies. The most striking result is that, below EW(Hb) = 100Å, Wolf-Rayet galaxies show an elevated N/O relative to non-WR galaxies. We interpret this as a rapid enrichment of the ISM from WR winds. We also show that the model predictions for WR features strongly disagree with the observations at low metallicity; while they do agree quite well with the data at solar abundances. We discuss possible reasons for this and show that models incorporating binary evolution reproduce the low-metallicity results reasonably well. Finally we combine the WR sample with a sample of galaxies with nebular He II 4686 to show that, at 12 + log O/H < 8, the main sources of He II ionising photons appears to be O stars, arguing for a less dense stellar wind at these metallicities, while at higher abundances WN stars might increasingly dominate the ionisation budget.