The Test Collection at ETS is a library of more than 25,000 tests and other measurement devices that makes information on standardized tests and research instruments available to researchers, graduate students, and teachers.
Although no longer kept up-to-date, this web site for the Eric Clearinghouse on Assessment and Evaluation still provides useful information on assessment, evaluation, and research methodology.
Edutopia, a web site of the George Lucas Educational Foundation, has a number of articles related to assessment. Includes articles, research, video clips, and resources.