For high-quality open-source corpora, common options include: Common Crawl (extremely large but noisy, requiring your own cleaning and filtering), Wikipedia Dumps (clearly structured and with good multilingual coverage, suitable for extracting entity relationships), the open-access subsets of arXiv and PubMed Central (high-density text in scientific fields such as mathematics, physics, and biomedicine), and Project Gutenberg and the Perseus Digital Library (important sources of historical languages and classical literature).