Links

marian.org This is the site of the Marians of the Immaculate Conception, who manage much of the Divine Mercy message and devotion, as, in 1941, the devotion was brought to USA from Poland by Fr. Jarzebowski, a member of the order. They have large, good quality posters of some of the Divine Mercy image versions for only 15[$]. They also sell large framed canvas images for churches.
The Devotion These sites contain very important information regarding the various elements of the Divine Mercy devotion including: The Feast of Mercy, The Chaplet of Divine Mercy, The Novena to the Divine Mercy, The Divine Mercy Image, The Hour of Mercy, Works of Mercy, Spreading the honor of Divine Mercy.

Oblates of Divine Mercy
http://www.geocities.com/kosmicphun2/divinemercy.html
http://www-rucker.army.mil/chap/divine_mercy.htm
http://www.rosaryworkshop.com/PRAYERdivineMercy.htm
The Congregation of the Sisters of Our Lady of Mercy This is the congregation that St. Faustina belonged to. This site has so much information that you won't find anywhere else. It seems to only be in Polish though. It seems that they have another site in English, but it doesn't contain the same info.
The Congregation of the Sisters of the Merciful Jesus This is the congregation spoken of in the diary which Jesus desired to establish through St. Faustina! It has come into existence! But for some reason it hardly gets any mention! Read all about it's amazing history! This is one of the best sites I have found regarding Divine Mercy. There is amazing, in-depth information on this site which you should know that you can't find anywhere else.
The Divine Mercy Movement of San Diego
(no website)
These wonderful souls sell framed Divine Mercy images at low prices. You just can't beat their prices. They also sell many other Divine Mercy items. Contact Carol and Joseph Pallazola: (619) 276-6637, 1905 Chicago Street, San Diego, CA 92110. 
Divine Mercy image and the Shroud These sites have information regarding the amazing match of the original Divine Mercy image and the Shroud of Turin. The International Life and Mercy Crusade, and "A Divine Coincidence?"by Fr. Seraphim Michalenko.
The Diary indexed by subject http://www.catholic-forum.com/catholicteacher/divinemercy1.htm