A number of publishers provide online updates to key textbooks as well as other content to help those studying law keep on top of the latest developments in the field.
The following are those I've identified to date:
Cavendish Publishing - provide updates to their key legal textbooks, and include links to useful web sites on various legal topics. Some titles, for example Hudson on Equity & Trusts also include podcasts which introduce aspects of the subject covered by the book.
Hart Publishing - their web site contains brief updates for some of their best known titles, such as Simester & Sullivan on Criminal Law.
Oxford University Press - OUP's Online Resource Centres are available for a number of student textbooks. At their most basic they include updates to the law since the book was first published, and provide links to useful web sites on the subject. Some titles go beyond this, for example Poole's Textbook on Contract Law also includes a section on how to answer problem questions and provides a number of test questions and answers.