Voyant Tools is a web-based text reading and analysis environment. It is a scholarly project that is designed to facilitate reading and interpretive practices for digital humanities students and scholars as well as for the general public.
This web-based software compares two text files to calculate the similarity between them.
The tidytext R package uses the design philosophy of the popular tidyverse package ecosystem in R and applies it to common textual analysis tasks.
This is the CRAN Task View for natural language processing. You can use it to find links to a multitude of R packages that can be used to process and analyze text.
Python's Natural Language Toolkit is among the most popular packages for working with textual data in Python. It provides the access to libraries for classification, tokenization, stemming, tagging, parsing, and semantic reasoning, among other tasks.