SEO Content Research

freemium software model and their website auditor has a great content editor tool

content marketing module is great (7 day trial), polished topic analysis that includes social shares/backlinks which is the content marketer's KPIs for success

can be used for free (just sign up for a free Google ads account) to find search volumes, but these can be useless unless you know/understand the search intent of the keywords

Downside is it doesn't indicate the search volume in any way to help you prioritise