Accessibility has always been a consideration for websites because if your users can't access the information the website isn't doing its job. But not all users can relate to a website in the same way, perhaps because of physical or educational disability. It used to be that website developers could just try to adhere to the Web Accessibility Initiative embodied in the W3C guidelines, but things have got more complicated.
The DDA (Disability Discrimination Act) has been replaced by The Equality Act 2010 according to the RNIB who are currently suing BMI Baby over poor accessibility of its site.
If you are developing government sites then you might be interested in work done for Broxtowe Council by the Shaw Trust, who employ disabled people and were used to test the council's web site over 60 hours. Broxtowe's site has just won the Shaw Trust Plus Accessibility award.
If you have educational establishments as part of your portfolio, they have extra accessibility issues since they have to consider The Special Educational Needs and Disability Act (2001). Durham University's web page gives some pointers for this area.
If you are developing pharmaceutical sites then you might be interested in the article by Nick Austen and Ben Deebie-Rogers, Legal Issues for Pharmacy Websites, as pharmacists have their own code of practice along with the e-commerce directives for selling online, the distance selling regulations and security data protection issues.
For more general info about legal requirements see The COI (Central Office of Information – but it will close on 31st March). And be aware that ethical requirements as well as legal requirements are beginning to find favour.