AntConc
AntConc is a downloadable tool for collocation (analysis of words that appear together), word frequency analysis, and more.
ConText
ConText is a downloadable tool for part-of-speech tagging, network analysis, topic modeling, and more.
Voyant Tools
Voyant is a web-based platform with many text analysis tools, that include visualizations. It is recommended for beginner use, as the interface is intuitive and rich visualizations make it easy to interpret a text corpus.
Antconc Tutorial on the Programming Historian
Lessons on the Programming Historian provide detailed walkthroughs on various tools. Antconc (mentioned above) is a useful tool for more in-depth analysis.