Start with 10 Great Alternatives to the Google Keyword Research Tool for a list of free and fee-based tools. For extensive keyword search statistics, KeywordSpy and KeywordDiscovery offer paid subscription services as well as a free trials.
A good overview is Optimizing Your Website: Ten Powerful SEO Tools by TJ McCue. Other useful SEO resources are:
Blogs worth following:
John Jantsch, founder of Duct Tape Marketing, has written a handbook entitled 7 Apps That Will Change The Way You Do Marketing (registration required).
In affiliate marketing, a percentage of the revenue from purchases made at selected online merchants will come back to your site if you display a text, banner, or other link to that merchant. Many affiliate marketing programs may be found at LinkShare.
More guidance and tools for affiliate marketing is available at Affilorama.
Consult MarketPosition: Search Marketing Resources for additional advice and practical tips, or Browse Website Magazine by Topics. Then, there are also Articles by Topic from the Digital Web Magazine (which ended its publication run at the end of 2008).
Finally, you can browse Email Marketing Resources Tools, White Papers, Guides, Webinars and Articles from Lyris.