I think it is always good to share lists of resources that can be helpful. I have personally used at least one of the tools in each of the categories.
Alerts and Trends
Content Marketing Ideas
Email Marketing and Marketing Automation
Images and Graphics
Native Ad Networks
Organization and Productivity Tools
SEO Analytics Tools
Social Media Tools
Website A/B Testing Tools
I want to add to this list as much as possible, so if you have any tools that you like and use, please add them to the comments and I can update the list.