SwipeWell - The only tool dedicated to helping you save, organize, and reference marketing examples (so you never feel stumped).
Validated ideas - List of 5,000 poorly executed but popular apps
AppGrowthLab (by SplitMetrics) - 55 app growth lessons from 30 app marketing experts
The 25 Days of Ideas - Free resources for coming up with startup ideas
Marketing Examples - A gallery of real world marketing case studies. Updated every day.
IdeasAI - OpenAI-powered startup idea generator