I have done some research on SEO/Internet marketing resources. I came across a few of them used by many people, let me say, successful internet marketers.
Hopefully, you’ll find them useful/helpful for your online activities. Mind, that for some of these resources a paid subscription is needed, but you may as well use the free range of services they offer.
Keyword Research
1. Google AdWords
2. Wordtracker
3. Ubersuggest
4. Latent Dirichlet Allocation Content Optimizer
Competitive Keyword Analysis
1. SpyFu
2. SEMRush
Backlink Investigation
1. Open Site Explorer
2. Majestic SEO Site Explorer
3. Ahrefs Site Explorer
4. Link Diagnosis
5. Open Link Profiler
Website Crawling & Link Checking
1. Screaming Frog SEO Spider Tool
2. Xenu’s Link Sleuth
3. W3C Link Checker
Content & Markup Analysis
1. Copyscape
2. W3C Markup Validation Service
PR & Outreach
1. HARO – Help a Reporter Out
2. Blogger LinkUp
3. DoubleClick Ad Planner