The North American Council for Online Learning offers links to a number of useful resources on K12 online learning, notably a good, current (late 2007) survey of state policies and developments.
The U.S. Department of Education has studied models for connecting students to advanced learning online.
The National Education Association offers a thoughtful guide to issues in online learning.
The Instructional Technology Council is a vendor of information about online learning and offers links to resources ranging from surveys to technical design considerations.
The Southern Regional Education Board has created an electronic campus which includes resources for high school students seeking advanced learning opportunities. The SREB also coordinates collaboration among state virtual schools within its area and offers valuable resources for the development of online schools.
K12 Inc., perhaps the leading independent vendor of online learning products for the K12 level, offers links to a number of white papers.
WorldwideLearn purports to be the premier directory for online education resources. However, its listing of high school offerings appears to be quite partial.
The Sloane Consortium is devoted to expansion of online learning, primarily at the post-secondary level.
The Innosight Institute looks at the process of education technology change through the lens of other industry experiences.
The Concord Consortium is a group dedicated generally to the exploration of the potential of technology in education.
The Open Courseware Consortium promotes open courseware, mostly at the university level. Open courseware may not fully support independent learning. It may just be lectures and course materials online. For example, MIT has made many of its course materials available online, is increasingly interested in supporting K12 learning, and has prepared a special port for high school students.
Education Week follows learning technology heavily. (You may need to register for free viewing of the collected materials on technology at this site.) Massachusetts shows up as relatively low graded on access to technology; this ranking does not look at use of online learning per se. As to online learning, Ed Week reports on states struggling with issues such as online charter schools.
U.S. News and World Report offers articles but not too much depth on online learning.