Wonderful free resources for finding definitions and alternative words. Watch out for American spellings when writing for an Australian audience.
http://www.merriam-webster.com
http://www.thefreedictionary.com
Looking for some fun with words? Search for phrases and slang that fit your theme.
http://idioms.thefreedictionary.com
A wealth of information about grammar and writing.
http://www.quickanddirtytips.com/grammar-girl
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.
https://online.maryville.edu/online-doctorate-degrees/grammar-resource-guide/