The Doctor in Prisoner of the Daleks
The Discontinuity Guide
In 1995, Paul Cornell, Martin Day, and Keith Topping published The Discontinuity Guide, a guide to all sorts of things about the TV series Doctor Who. Subsequently, another website, The DiscContinuity Guide, has used the format to cover Big Finish's monthly Doctor Who audios.
This site aims to cover almost everything not covered by the other two guides. This includes the new series, the books, the comic strips, and various Doctor Who spin-offs.
A separate page explains the Discontinuity Format.
There are also a number of Boxed Sections that expand on particular continuity issues.