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.
Summary: Generate Ideas Through “Object Writing”
Word of the Day, plus user forums: objectwriting.com
Dictionaires
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
Software
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.
Chord Finders
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.
