I have downloaded from this link a quite poor quality scan of the letter dating May 1966 that Grothendieck sent to Tate mentioning his ideas about generalizing Monsky-Washnitzer cohomology. I am trying to put it in LaTeX, at least for my own reference. So, first of all, do you know if it ever appeared in print somewhere or if the typing has already been done by someone? And, secondly, does anyone has a better-quality file? The big problem with this is that many top and bottom lines are missing, sometimes making it impossible to reconstruct the original sentence.
In the light of past events ("Les Archives Grothendieck"), we now have an "ameliorated" version of the letter:
Cote n° 6. Lettre Tate (mai 66) (Cristaux) : lettre (1966), tapuscrit, notes manuscrites. 1966 - [à partir de 1970].
See [here] for more.
I don't know if this is of any use.... but I believe that the ideas in this letter were written up somewhat later in this article:
Grothendieck, A. Crystals and the de Rham cohomology of schemes. 1968 Dix Exposés sur la Cohomologie des Schémas pp. 306–358 North-Holland, Amsterdam; Masson, Paris
The article is actually some notes by Coates and Jussila of a seminar by Grothendieck. You can find a copy of the article at chapter IX of this book.