The RapidAPI blog post from January 2021 highlights the top seven Natural Language Processing (NLP) APIs available on its marketplace, which were chosen after reviewing over 53 options. These APIs, designed to enhance the interaction between computers and human languages, include the Aylien API, Text Summarization API, Twinword Text Analysis Bundle API, IBM Watson Alchemy API, RxNLP API, Linguakit API, and Geneea Interpretor NLP API. Each API offers unique capabilities such as sentiment analysis, text summarizing, and information retrieval, catering to various NLP needs. The blog post also outlines the popularity scores and specific strengths of these APIs, emphasizing their utility for developers in processing and analyzing large amounts of natural language data.