Amici Thomae Mori
The Friends of Thomas More, linked with the Center for Thomas More Studies at the University of Dallas (see below) which edits and issues its learned review, Moreana, but serving a more general audience with its events and newsletter, the Gazette Thomas More.

Catholic Encyclopedia of 1917
Useful introduction to the stories of the Catholic martyrs, but without critical treatment and with a non-ecumenical spirit from times prior to the Second Vatican Council

Catholic League (UK)                        
An ecumenical society promoting the Catholic ecumenical movement and the corporate reintegration and reunion of all Christians in communion with the apostolic see of Rome. For many years, the League has been promoting reparation for the sins of the disunity of the Church in England and the historical mutual martyrdoms, as well as reconciliation and healing of memories.

Center for Thomas More Studies
At the University of Dallas, Texas

Centre for Reformation & Early Modern Studies
Based at the University of Birmingham. There is a superb links and online resources page.

Congregation for the Causes of Saints
The Vatican department charged with the process and history of establishing the veneration of the Saints, the Blessed and the Servants of God.

Hagiography Circle
The Hagiography Circle is a body of young scholars bound by a common interest in “re-telling” the lives of contemporary models of holiness who, within the past seven years, have dedicated some of their time to reading, translating, and reflecting on biographies sent in by promoters of beatification and canonization causes.

Hagiography Society of America
The Hagiography Society was founded in North America in 1990 to promote communication among scholars in various disciplines whose research involves the study of early Christian or medieval saints' legends.

John Foxe's Book of Martyrs
Online critical edition of all versions, from the Humanities Research Institute, University of Sheffield

Société des Bollandistes
A scholarly association of Jesuits and others, based in Belgium and continuing work begun in 1607, dedicated to the critical study of Christian hagiography. 

Society for Ecumenical Studies
Promoting dialogue, mutual understanding and research, and receptive ecumenism.

Society for Reformation Studies
Based at King's College London

Tyburn Convent
The shrine to all the Catholic martyrs of the Reformation period near Hyde Park in London. The Benedictine community has been concerned to ensure that the commemoration of these martyrs also takes into account the whole history of the Church in England and remembers with sorrow, yet with hope for unity, those Christians who were Anglicans and Protestants who also lost their lives in faithfulness to Christ.

Ample information on the life of many of the martyrs mentioned on this site