What are some guidelines and examples for designing usable legal terms for web sites? I know that most users don't read it, but are there innovative ways to improve readability and presentation of these (traditionally) boring documents?
Not an answer based on studies but on personal experience, and some research I did on this topic recently:
Some general aspects of Terms and Conditions:
Apart from that there is another aspect that seems to me widely ignored by most companies: Legal documents can also be used to increase trust. Especially in eCommerce it's vital to the success of the business that customers trust your site and business. Companies spend tons of money for trust seals on the one hand but might scare away those customers who look up the companies' legal docs. Imagine the power of a clearly understandable document giving you a good feeling that company is not trying to fool you.
But is this really a problem? Probably not significant. In my experience (based on looking at web analytics data) less than 2% of the users look at these legal documents. But then ask somebody dealing with conversion optimization in eCommerce what it takes to achieve a 1-2% conversion uplift. And why not take this as a chance to score against your competitors?
What can we do for better legal documents, to increase readability and also trust?
 Not saying what but how little users read and that it's only 10% of longer documents http://www.useit.com/alertbox/percent-text-read.html
 Plain English instead of legal jargon at Google http://www.google.com/policies/terms/ (I am aware they also got lots of criticism for this, mostly for being vague)
 Good example that seemed to become popular recently is http://500px.com/terms (even though their terms are linked on in the footer and hard to get to as the site uses Infinite Scrolling)
Edit March 2013:
Just stumbled upon an interesting TED talk ranting about legal jargon in legal to non-legal conversations, contracts etc. and wanted to share it here: http://www.ted.com/talks/alan_siegel_let_s_simplify_legal_jargon.html
As far as placement, depending on your industry, it may need to be a link in the footer of every site page. Giving it a dedicated page rather that a modal/pop-up is good for SEO.
As far as structure, write it like any other well-designed document. Bold for headings, paragraphs chunked and grouped in logical order, contact information.
Example from former company: