The Van Gogh Museum in Amsterdam, in conjunction with the Huygens Institute, has digitized all 902 known letters written by and to Vincent van Gogh. You can do a keyword search as well as search by period, correspondent, place, or those with sketches. While you can view a scan of each letter, the site also offers a transcription of the original language (as paragraphs or by line count), an English translation, and notes and images to particular works of art when referenced.
This endeavor, unveiled by HM Queen Beatrix on Wednesday, is also in book form and corresponds with the Museum’s exhibition “Van Gogh’s Letters: The Artist Speaks” (October 9, 2009-January 3, 2010)