Free SEO tools that you might find useful for various aspects of search engine optimization. Keep in mind that the popularity and availability of these tools can change over time, so it's a good idea to verify their current status before using them:  

1. Google Search Console: Monitor and improve your website's presence in Google search results.
2. Google Analytics: Track website traffic and user behavior.
3. Google Keyword Planner: Research keywords for your content and campaigns.
4. Ubersuggest: Keyword research, backlink analysis, and content ideas.
5. Yoast SEO: WordPress plugin for on-page optimization.
6. SEMrush: Limited free access to competitive research and keyword analysis.
7. Moz Free Tools: Includes keyword research, link analysis, and more.
8. AnswerThePublic: Generate content ideas based on search queries.
9. Screaming Frog SEO Spider: Crawl and analyze websites for SEO insights.
10. Keyword Surfer: Get search volume and keyword suggestions in Google search results.
11. SERPsim: Preview how your page might appear in search results.
12. GTmetrix: Analyze website speed and performance.
13. Bing Webmaster Tools: Monitor your website's performance in Bing search.
14. Varvy SEO Tool: On-page optimization analysis and tips.
15. SEOquake: Browser extension for SEO metrics.
16. Beam Us Up: Free crawling tool for technical SEO analysis.
17. Robots.txt Generator: Create a robots.txt file for your site.
18. Varvy's Mobile-Friendly Test: Check if your site is mobile-friendly.
19. CanIRank: Keyword difficulty analysis and competitive insights.
20. Browseo: View your site as search engines see it.
21. Schema Markup Generator: Create schema markup for your website.
22. LinkMiner: Chrome extension for finding broken links.
23. Hunter: Find email addresses associated with a website.
24. SEO Web Page Analyzer: On-page optimization analysis.
25. Google Trends: Track search term popularity over time.
26. Keywordtool.io: Generate keyword ideas for SEO and PPC campaigns.
27. Majestic SEO: Limited free access to backlink analysis.
28. Rank Math: WordPress SEO plugin for on-page optimization.
29. BuzzSumo: Discover popular content and influencers.
30. Check My Links: Chrome extension to find broken links on a page.
31. Copyscape: Check for duplicate content.
32. Wordtracker Scout: Chrome extension for keyword research.
33. Link Explorer (by Moz): Analyze backlinks and domain authority.
34. Google PageSpeed Insights: Analyze and optimize your site's performance.
35. Ahrefs Backlink Checker: Check backlinks to any domain.
36. SEO-Browser: View web pages without CSS and JavaScript for SEO analysis.
37. SEObility: On-page SEO audit and backlink analysis.
38. Broken Link Checker: WordPress plugin to find broken links.
39. Siteliner: Find duplicate content and broken links.
40. Keyworddit: Extract keywords from Reddit.
41. Seed Keywords: Gather keyword ideas from your friends.
42. Rapid SEO Tool: Windows software for on-page optimization.
43. Siteliner: Check for duplicate content and broken links.
44. Lighthouse: Analyze and improve the quality of web pages.
45. Google My Business: Manage your local business appearance on Google.
46. SEO Site Checkup: On-page and technical SEO analysis.
47. SEMrush SEO Writing Assistant: Improve content quality and SEO.
48. SEOquake: Analyze on-page and off-page SEO elements.
49. Keyword Sheeter: Generate keyword ideas in bulk.
50. BrowSEO: View web pages like a search engine.
51. SEOptimer: On-page and technical SEO analysis.
52. Siteliner: Analyze website content for duplicate content and broken links.
53. Keyword Tool: Generate keyword ideas from various sources.
54. Xenu's Link Sleuth: Find broken links on your website.
55. Lipperhey: On-page SEO analysis and optimization tips.
56. Bulk Google Rank Checker: Check your website's rankings in bulk.
57. Keyword In: Discover keyword opportunities.
58. Google Keyword Planner: Research keywords and get search volume data.
59. SimilarWeb: Analyze website traffic and user engagement.
60. Keywords Everywhere: Chrome extension for keyword research.
61. Google Analytics Solutions Gallery: Access pre-built reports and dashboards.
62. SERPSim Snippet Generator: Create and preview title and meta description tags.
63. Ahrefs Toolbar: Chrome extension for SEO metrics.
64. SEMrush Backlink Checker: Analyze backlinks to a domain.
65. XML Sitemaps: Generate XML sitemaps for your website.
66. OpenLinkProfiler: Analyze your site's backlinks.
67. Rank Checker by SEO Review Tools: Check keyword rankings.
68. Seobility Browser Extension: Analyze on-page elements.
69. WooRank: On-page and technical SEO analysis.
70. Yoast Duplicate Post: Duplicate pages or posts in WordPress.
71. W3C Markup Validation Service: Check HTML markup validity.
72. Google Data Studio: Create customized reports and dashboards.
73. Google Mobile-Friendly Test: Check if your site is mobile-friendly.
74. Google Optimize: Test and optimize site content.
75. SERPROBOT: Track keyword rankings.
76. Beam Us Up SEO Crawler: Crawl websites for SEO insights.
77. Yandex Metrica: Web analytics platform.
78. Anchor Text Over Optimization Tool: Avoid anchor text over-optimization.
79. Schema Markup Generator: Create schema markup code.
80. Bing Keyword Research: Keyword research tool by Bing.
81. Google Correlate: Find search patterns that correspond to real-world trends.
82. SEO Hero Toolbar: Chrome extension for SEO metrics.
83. StatCounter: Track website traffic and user behavior.
84. Ahrefs WordPress SEO Plugin: On-page optimization and analysis.
85. SEO Panel: Open-source SEO control panel for managing multiple sites.
86. GTmetrix: Analyze page speed performance.
87. SEO Review Tools: Access a suite of various SEO tools.
88. Structured Data Markup Helper: Create structured data markup.
89. Alexa Rank Checker: Check Alexa rankings for websites.
90. WordStream's Free Keyword Tool: Generate keyword ideas.
91. Ahrefs Keyword Generator: Generate keyword ideas.
92. XML-Sitemaps.com: Generate XML sitemaps for your site.
93. SEO Minion: Chrome extension for SEO tasks.
94. Rank Math: WordPress SEO plugin.
95. Lighthouse: Analyze web page quality.
96. Yandex Webmaster: Manage your site's presence in Yandex search.
97. Moz Local Check Business Listing: Check your local business listings.
98. Keyworddit: Extract keywords from Reddit.
99. Google Disavow Tool: Disavow unwanted backlinks.
100. BuzzStream's Meta Tag Extractor: Extract meta tags from web pages.

  Always check the latest information before using any tool. Additionally, some tools might have limitations in their free versions and may offer more features in their paid versions.