 Les Dérivateurs
Grothendieck's 1991 manuscript `Les dérivateurs'.

Scans of Grothendieck's prenotes for EGA 5 (171 pages)
The prenotes were given to Piotr Blass by Grothendieck in 1987. The
pages are somewhat mixed up so start by consulting the
Table of Contents. See also the 1987 letter from Grothendieck to
P. Blass in the list of letters below. Links to the version of this
part of EGA typed, revised and published by P. Blass and S. Klasa
can be found on the published mathematical texts page.
Mathematical Letters (some later published, some not)

Motives
letter to Serre (August 8, 1964) The English translation of a letter
from from Grothendieck to Serre which contains his first written mention of
motives. The original letter is published in Correspondance
GrothendieckSerre, Documents Mathématiques, Société
Mathématique de France, 2001.

Crystals letter to Tate (May 1966) 
A 31page letter from Grothendieck to Tate, in which the
notion of crystals was born and baptized (with a nice
``commentaire terminologique'' about the name).
It contains the conjectures, based on an analogy with the
work of MonskyWahnitzer in the affine case, on the
comparison between crystalline and de Rham cohomology.
For a firsthand historical survey of the theory (starting
with the present letter), see the article by L.Illusie
in Motives, Proceedings of Symposia in Pure Mathematics,
Volume 55, 1994, Part 1, 4370.

Motives
letter to Illusie (May 3, 1973) A letter from Grothendieck to Luc Illusie
containing remarks on motives. The original letter is published as an appendix
to an article by Jannsen, in Vol. I (p. 296) of
Motives (U. Jannsen et al. eds), Proc. Symp. Pure Math 55, AMS, 1994.

Scans of three letters from Grothendieck to Larry Breen containing
remarks on the yoga of homotopy (ncategories etc).
Letter 1
(February 17, 1975, 5 pages)
Letter 2 (July 1719, 1976, 22 pages + 4 pages of comments)
Letter 3 (April 8, 1982,
10 pages, page 4 unfortunately missing)
The same letters in a smaller size suitable for printing:
Letter 1,
Letter 2,
Letter 3,

Anabelian
letter to Faltings (June 27, 1983)  A letter from Grothendieck to
G. Faltings in German, describing Anabelian Algebraic Geometry.
To Grothendieck's disappointment, Faltings never responded to this letter.
However, Faltings' student Shinichi Mochizuki picked up the subject years
later and proved Grothendieck's anabelian conjecture for hyperbolic curves.
This letter was later published in Geometric Galois Actions I (P. Lochak and
L. Schneps, eds., London Math Society Lecture Note Series 242, Cambridge
University Press (2000) ).
(English
translation) (
Scan of the original)
 The 1987 letter from Grothendieck to P. Blass accompanying
the prenotes for EGA V.