MIT's online edition of Hamlet
http://www-tech.mit.edu/Shakespeare/hamlet/index.html
You should certainly read Hamlet in an edition with copious annotations and notes, none of which is available in MIT's free online version of the play. However, this online edition is very useful in performing searches and other actions. It makes it very easy, for instance, to trace Shakespeare's use of a single term or phrase throughout the play. For example, do a search for "kind" and see how often it pops up in the play, and how variously Shakespeare uses the term.
Hamlet Online
http://www.tk421.net/hamlet/hamlet.html
A very useful compendium of links and articles having to do with Hamlet.
Hamlet and His Problems by T.S. Eliot
http://www.bartleby.com/200/sw9.html
Eliot's very famous and worthwhile take on the play. Much later criticism has been influenced by this particular essay.
Dr. Freud's Hamlet
http://arts.ucsc.edu/faculty/bierman/Elsinore/Freud/freudSolved.html
A quick and dirty summary of one of the most famous interpretations of Hamlet: Freud's reading of the play in light of the Oedipal Complex.
Samuel Johnson -- Notes on Hamlet
http://shakespearean.org.uk/ham1-joh.htm
Samuel Johnson's eighteenth-century edition of the works of Shakespeare guided the course of scholarship for centuries. His notes are still informative and entertaining.
Shakespeare and His Critics
A lovely compilation of criticism throughout the centuries.