Ubersuggest's free tier lets you do keyword research, site audits, and backlink checks, but it's capped on daily searches and features. Still, it's solid for starters, and their paid plans are like 90% cheaper than Ahrefs or SEMrush if you upgrade.
AnswerThePublic is killer for digging up content ideas from search questions and phrases. Free plan gives you a few searches a day (1 without signup, 3 with), pretty limited but enough to spark ideas. Paid version's affordable too compared to the pricey competitors.
Google Search Console is totally free and a must—tracks your site's performance, indexing issues, and search queries. No limits really, but it's more for monitoring than deep research.
Ahrefs Webmaster Tools gives you free site audits, keyword ideas, and backlink analysis for your own sites. It's limited to verified domains and no competitor spying, but way cheaper to upgrade than their full suite.
Google Keyword Planner is free if you've got a Google Ads account—great for keyword volume and ideas. It's ad-focused so estimates can be broad, but no cost and integrates easy with other Google stuff.