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.
Other
New Testament Study Helps: Mark's Gospel
An analysis of the Book of Mark which provides historical information, a short description of the book along with details about the possible author.
Christian Classics Ethereal Library
World Wide Study Bible: Mark
Provides 50 different resources on the New Testament Book of Mark, including scripture, audio scripture, commentaries and so on.
Bible Study Tools
John Darby's Synopsis of the New Testament: Mark
A commentary by John Darby on the Book of Mark. It includes a thorough discussion of each chapter. It is scholarly and has some interesting things to contribute.
Bible Study Tools
The 1599 Geneva Study Bible: Mark
This is a very scholarly commentary on the Gospel of Mark. This would be interesting to compare with modern translations and is a good resource to use as part of a word or in-depth study.
Bible Study Tools
People's New Testament: Mark
This is a great basic site that anyone can use. It includes a commentary on each chapter complete with many linked cross-references.