Just over 900 of Vincent van Gogh's letters have been digitised and put online in Vincent van Gogh: The Letters. A digital accompaniment to a print publication, the letters are fully transcribed, translated and searchable.
The strongest feature of the site is that way that you can view a single letter in multiple ways - an image of a page, the transcription, the original line breaks, the English translation, the editors' notes, and related works. You can also easily compare and contrast, as shown below.
There are amazingly detailed notes about the publication (down to the insertion of missing commas) and some solid info about the technology behind the site.
And in other web news: changes to the display of MoMA's permanent collection (including frame removal).