I have started learning about search engine optimisation recently. These are the key tools I use:

a) SEOMOZ (I have a free account). Use the SEO tools periodically to try to understand what is going on (particularly Open Site Explorer)

b) Hubspot. I tried for 30 days, but prefer the freebies: Website grader | Blog grader

c) SEObook. This sends out a bunch of free emails and e-book that, if time permits, are worth reading.

d) Search Engine Guide.

e) WebCEO. Just trying it out now.