Word segmentation, part-of-speech tagging, and more:
Check out the Center for Corpus Development, NINJAL for links to many corpora and databases. Here are some notable corpora.
WordNet, including synsets (synonym sets) only, has been created for Japanese. Please visit the page to download the sqlite3 database of Japanese WordNet, then use one of the APIs in a variety of programming languages to use it in your own code. Here is a link to the Python API for Japanese WordNet.
These are plain-text files (compressed in zip format) of a couple of NINJAL corpora, segmented with spaces between words to be used with software for Western text analysis like MALLET, Topic Modeling Tool, and Voyant.