Source: 4 Essential SEO Infographics from 2009, by Rand Fishkin | SEOmoz
Stashed in: Venn!, Infographics!, startup, Wikipedia
Even though Rand Fishkin made this scatterplot three years ago, it's still quite relevant.
The highest value things you can do are keyword research, on-page optimization, embedded widgets and badges, and viral content.
I also enjoyed this Venn Diagram of what makes a great SEO professional:
Wikipedia links help?!