Wonderful free resources for finding definitions and alternative words. Watch out for American spellings when writing for an Australian audience.
Looking for some fun with words? Search for phrases and slang that fit your theme.
A wealth of information about grammar and writing.
Keyword checker: http://smallseotools.com/keyword-position/
Plagiarism checker: http://smallseotools.com/plagiarism-checker/
Keyword planner: https://adwords.google.com/KeywordPlanner
(You will need to set up an Adwords account to access Google's keyword planner.)
For budding young authors or writers, this website has a wealth of information. Watch out for American spellings when writing for an Australian audience.