University of Toronto (Canada)
University of Toronto: Five Gospel Parallels
Unbelievable site which has all five gospels on the page at one time so you can do a text by text comparison. Very interesting and helpful.
New Advent
Catholic Encyclopedia: The Gospel of Matthew
This is a scholarly explanation of the origin and content of the Gospel of Matthew in the New Testament. Please note that "The Catholic Encyclopedia" is a historic reference source and should be viewed in the context of resource material...
Other
New Testament Study Helps: Matthew's Gospel
An analysis of the Book of Matthew which provides historical information, a short description of the book along with details about the possible author.
Luther Seminary
Bible Tutor: Matthew
This Lutheran website features a summary of the Gospel of Matthew.