There are a lot of songwriting resources on the web, below is a list of some of my favorites that I use regularly. By no means is this a complete list, use Google to find more. If you have a resource that works for you, feel free to contribute in the comments.
Object Oriented Writing
I first read about Object Oriented Writing in the book, "Writing Better Lyrics" by Pat Pattison. Basically, this is a 10 min a day exercise to help your writing.
Word of the Day, plus user forums: objectwriting.com
RhymeZone.com is one of the most comprehensive online tool to discover rhymes, synonyms, definitions, descriptive words, antonyms,
The Online Slang Dictionary - American, English, and Urban slang
To be honest, I've tried Songwriter's Companion, but find that a notepad, Pages or Word along with GarageBand works better for me.
Songwriter's Companion - For Mac, one time fee.
MasterWriter - All platforms, monthly subscription.
NoteWorthy Composer™ - Music notation software for Windows. Try/Buy.
Chord Progression Generators
It's easy to get into a musical rut. These sources help you explore different keys and moods to put some life into your typical I IV V.
AutoChords, although they have apps for iOS and Android, their best work is on the web. Guitar/Piano.
ProgressionGenerator. Similar to AutoChords. Also, check out the Scale generator and read their About page, which is music theory 101.
Need to identify what chord you're playing? Or look up a chord in an alternate tuning??
Chorderator.com - In addition to reverse chord look up, there's a great chord library that has alternate tunings, mandolin, bass and even an area to enter your own tuning. This is awesome.
JamPlay - Chord Finder / Chord Namer. Additional tools seem to be premium, but this is a great site.
Alternate Tunings - Information/essays on alternate tunings.