SEO & Keyword Research
Google Keyword Planner: Nothing like searching for keywords using the tool created by big G himself.
Semrush: The very best SEO tool available. Research on your competitors, do keyword research, and much more.
Long Tail Pro: Find Long-tail keywords with this software.
Ubersuggest: Google suggestion on steroids – find lots of keyword suggestions for a given keyword!
WordPress SEO by Yoast: Best SEO plugin for WordPress users.