Menachem Mendel Kasher

Menachem Mendel Kasher was a Polish-born rabbi and prolific author. His major work, Torah Shelemah can be divided into two parts. The first part is the encyclopedia, the first work to publish all of the written law (the pentateuch) and the oral teachings (Talmud and Midrashim) side by side. He published from manuscript form several previously unknown midrashic works such as the Midrash Teiman. The latter part consists of the extensive annotations and addendum in which he uses his awareness of variant texts as well as his almost encyclopedic knowledge in all Jewish works to clarify many obscure points in the Talmud and Maimonides.

At the age of 19 he edited the periodical Degel Ha'Torah the mouthpiece of the Polish branch of Agudas Yisrael.

He was the editor of a Torah journal "Noam".

He helped bring the Rebb of Gur to Israel during WWII. He was the Rosh Yeshiva (Dean) of Yeshivas Sefas Emes of Gur in it's first two years.

Another work Gemarah Shelemah which was to have discussed and compared variant texts of the Talmud was never completed. He was awarded an honorary doctorate from Yeshiva University. He was awarded the Israel prize (1953).

Some of his Halachic ruling are:

Permited an eruv in Manhattan (against the ruling of R. Moshe Feinstein)
He formulated an Halakhic stance on the International Dateline in Jewish Law
He argued against the "Lieberman clause" as a solution to the Agunoh problem.(see Get (conflict))

He was the author of:

Torah Shlemah, a monumental encyclopedic work on the Torah;
Hatekufah Hagedolah, a book explaining the meaning of our times according to Judaism.
Ha'odom Al Hayareach a book discussing the theological and legal issues involved when the moon was explored
Ha'mechilta D' Rashbi V' HaRamabam discusses the relationship between Mishne Torah and this recently published Medrash
Divrei Menachem Responsa (includes responsa from many of the foremost scholars of the day including the Rogatchover Gaon and Kli Chemda)

Reference

(partially based on Hebrew Wiki)