The Spanish Tragedy
http://www.elizabethanauthors.com/span1.htm
Full text of the play online, complete with glossary and appendix.
Thomas Kyd (1558-1594)
http://www.luminarium.org/renlit/kyd.htm
Information on Kyd from Luminarium's extensive anthology of English literature, complete with biographies, works, and critical essays.
Kyd, Thomas
http://www.bartleby.com/65/ky/Kyd.html
The Columbia Encyclopedia's short entry on Kyd.