There are many online options for some of the writer or poet’s most basic tools- the word processor, dictionary and rhyming dictionary, and thesaurus.
Here’s information about a few of these tools.
Most computer users will be familiar with MS Word, a common word processor with many useful features. What some of us may not know is that recent updates of MS Word (probably editions from 2010 and later) have built-in synonym and dictionary features. Some of the older versions have a define feature, but it is called “Look Up”. It is likely the most feature-rich of the word processors I know of, but it is also a paid software.
One downfall of MS Word is that it does not automatically save your work. I can not count the number of times I have lost work irreversibly in the past when MS Word was my only option. I have spent more than my fair share of time retyping entire essays from scratch at the last minute. If you need help saving your work on MS Word, check out this tutorial: https://support.office.com/en-us/article/Save-a-document-in-Word-B7F55D8...
On the other hand, most online word processors have the benefit of being free (or just costing $1 - $2 USD) and automatically saving (the only downfall to that is you must be connected to the Internet for the auto-save feature to function, so any time you are out and about working on your tablet, laptop, or smartphone without a stable Internet/WiFi/mobile data connection, you won’t be able to save).
The most common free online word processor is Google Drive/Docs. It has somewhat less features overall compared to MS Word, but is a powerful tool for those who might otherwise obsessively click the Save button on MS Word! The great thing about this online word processor is that it is connected to your GMail account, so you can access it anywhere from any computer or device just by logging into your GMail account. There is also a Desktop/’Google Drive for Windows” feature which allows you to download Google Drive for your personal computer and syncs your documents for offline access.
Google Drive does not have a Synonym feature, but it does have an “Explore” feature. By right-clicking on a word and selecting “Explore ---”, you will bring up a pane that gives you web, image, and Google Drive search results of that word. You can do the same thing with an entire phrase by selecting/highlighting the phrase then right-clicking (learn how to select text in the above Copy and Paste tutorial).
If these particular apps are not your style, a simple/free notepad app or one of these note-taking apps may be better: http://www.gadgetmatch.com/best-note-taking-apps-phone-android-ios/
For those writers who want a simplified, clean, focused, and uncluttered writing/typing experience with less bells and whistles (maybe even something akin to old typewriters) try these: http://fieldguide.gizmodo.com/9-minimal-word-processor-apps-for-distract... or Ywriter4: http://www.spacejock.com/yWriter4.html
The online/digital options for online dictionaries and rhyming dictionaries are just as plentiful, if not more plentiful, than word processes (the above links only share some of the available ones). Google can define words for you without using an additional dictionary by simply searching the word with “define” such as “define alliteration” or “alliteration define” as the search terms.
English to Other Language Dictionaries
For an online rhyming dictionary, my go-to is RhymeZone. Rhymezone features a dictionary, rhyming dictionary, thesaurus, near rhymes, and many other features useful for poets like searching for specific phrases or searching for a usage of a word in Shakespeare. It has the added benefit of organizing the words it provides by syllable so that you can find the perfect word to fit a syllable count in your poetry.
For picking end, middle, double, or other types of rhymes, (and also learning about the many different types of rhyme) use Rhymer.
To generate random words by part of speech or other parameters, try one of these generators (great writing prompt):
I'm just your average fictional creature, living in a swampland by the sea.