Footers on web sites aren't what they used to be... these days they are detailed, content-heavy, and as action-packed as the site above it. Usually, these new-style footers at a minimum contain a site map, the typical range of social media link buttons, sign up for our newsletter, and contact details. For the most of us, this is visually an interesting development, but the reasons behind this trend are not so obvious.
One compelling reason to have a detailed footer is for search engine optimisation (SEO). However, when a footer is created primarily for this reason, it will have a ridiculous amount of content in it that seems quite nonsensical. Basically it looks like a cheap trick... so best to be avoided.
Overall, your footer should make it easy for people to navigate your site (even if they have scrolled down to the bottom of a very long article). It should make it very easy for them to contact you, and easy for them to investigate your other appearances on the web via social media. And it should be just as well designed as the rest of your site.
It will be pretty interesting to see if this "trend" continues, or develops and morphs into something else in the next 6-12 months.
Other articles on the topic worth reading: http://www.visualswirl.com/articles/fat-footers-the-long-tail-of-your-website/; http://www.pvmgarage.com/2010/01/trends-and-examples-in-recent-web-design-big-informative-and-creative-footers/