This may not apply directly to a lot of SEOs, but an automated regression analysis is what allowed me to build most of the SEOmoz tools' reports accurately. This sweet little page (watch out for frames) will allow you to plugin a data set and have an equation returned to "best match" the data. Infinitely useful for all sorts of applications, not least of which are ranking how valuable 500,000 links are vs. 50,000 (as reported by Yahoo's new site explorer).

Many thanks Messrs. Werner & Costenoble!