sfmarketing.cloud is a SFMC resource put together by Zuzanna Jarczynska which hosts a great amount of resources for all levels of SFMC developers from for loops to API to Marketing Cloud Connect.
McChat is a youtube series from Eliot Harper that aims to help developers get the most out of the SFMC platform. In these bite size videos Eliot provides insights into the various parts of SFMC.
InvokeCreate is a new SFMC Developer blog by Jason Hanshaw that aims to explore SFMC from inside the platform as well as externally. Jason has an interest in developing for SFMC with python.
Camroberts.com.au is a SFMC resource put together by the very talented SFMC user Cameron Robert. Cameron is a Salesforce Marketing Champion and covers topics ranging from IP warming to customer engagement strategy.
Mateusz has some great documentation, especially on SFMC SQL, that are an ongoing and living resource to help the 'up and coming developer' learn SFMC or as a reference for the more grizzled SFMC developer to use and ensure they are following best practice.
The AMPscript guide is one of (if not THE) most complete resource for all things SFMC AMPscript. It takes form as not only on-line searchable documentation but a hardcopy book as well. It’s more in-depth than the official SFMC documentation and comes complete with useful examples that assist in understanding of the functions. There is a cost associated with this resource to access the full documentation, but it is well worth it.
Api.mcexperts.ninja is a SFMC API resource that saves developers a ton of time! This is a collection of REST and SOAP API calls that can be imported and run through the Postman API tool. This is a great resource for anyone beginner to expert to get familiar with the SFMC APIs or work through building API payloads for production use!
SFMC For Dummies is a printed resource for all things SFMC. SFMC For Dummies takes you step-by-step through the SFMC platform, giving you both high-level overviews and in-depth understanding of the various tools within SFMC. This book is a great way for any email marketer that wants to gain some insight into SFMC without having direct access to an instance or is gearing up to take on work in a new part of SFMC.
The Trailblazer Community is a forum hosted by Salesforce for all of the wonderful Salesforce Trailblazers. It’s a great place to connect with others, share ideas, and collaborate
Salesforce.stackexchange.com is a community forum hosted by the developer community stackexchange.com only – for all things Salesforce. This is a great resource for seeking out answers to specific questions and topics.