BBC Extinction Files - http://www.bbc.co.uk/education/darwin/exfiles/
Broad, non-technical coverage of the whole field; a good introductory work. Includes mass extinctions, theories, the fossil record, FAQs, and a glossary.

Extinction - http://www.peripatus.gen.nz/paleontology/extinction.html
A number of extinction concepts are discussed and loosely defined, followed by a survey of some important extinction and "extinction-like" events, primarily from the Phanerozoic.

Extinctions: Cycles of Life and Death Through Time - http://hannover.park.org/Canada/Museum/extinction/tablecont.html
Covers mass extinctions, minor ones, patterns or periodicity of extinctions, and a citations page.

Extinctions: Cycles of Life and Death Through Time - http://park.org/Canada/Museum/extinction/tablecont.html
Information about the patterns of extinction, both the major mass extinctions and some smaller, less well-known events.

Mass Extinction - http://www.lassp.cornell.edu/newmme/science/extinction.html
A mathematical model for mass extinction.

The Late Devonian Mass Extinction - http://www.lpi.usra.edu/meetings/impact2000/pdf/3072.pdf
A professional technical paper discussing whether the Late Devonian (Frasnian-Famennian) mass extinction was initiated by an extraterrestrial impact or an earth-bound event.