The following is a list of resources for finding articles about teaching mathematics at the university level.
Books
Assessment Practices in Undergraduate Mathematics MAA Notes #49 (1999)
Ideas from mathematics education: an introduction for mathematicians, Alcock L. (2009)
Research in Collegiate Mathematics book series published by the AMS
Teaching Mathematcis in Colleges and Universities: case studies for today's classroom, Friedberg (2001)
Databases
Electronic Literature Database endorsed by the Special Interest Group of the MAA on Research on Undergraduate Math Education
List of Journals maintained by the MAA special interest group.
Mathematics Education Database MathEduc (formerly MATHDI) is an international database of literature on research, theory and practice in mathematics education
National Science Digital Library provides links to online educational resources for teaching and learning.
Springer's list of mathematics educations journals
TheProof Comprehension Research Groupat Rutgers University publications list.
Proof Preuve Prueba - The International News Letter on the Teaching and Learning of Mathematical Proof is a curated list of articles from a variety of journals.
Journals
The College Mathematics Journal (CMJ) is published by the MAA and has a variety of content aimed at instructors of first and second year courses. The articles are available on JSTOR and the topical index (see MAA website for current version) is particularly useful for finding interesting and relevant content. The mathematics department at Harvey Mudd also maintains a searchable database of the articles.
The Mathematics Education Research Journal (MERJ) is published by the Mathematics Education Research Group of Australia and is available through Springer. The content pertains to mathematics education at the primary, secondary and tertiary levels, and takes the form of full-length research articles and short reports.
Teaching Mathematics and its Applications is published by Oxford and focuses the improvement of mathematics teaching and learning for students from upper secondary/high school level through to university first degree level with an emphasis on applications.
Journal for Research in Mathematics Education published by the National Council of Teachers of Mathematics
Australasian Journal of Educational Technology published by the Australasian Society for Computers in Learning in Tertiary Education
International Journal of Mathematical Education in Science and Technology published by Taylor & Francis
Course Supplements
GoodQuestions Project - Calculus materials developed by Cornell University in an effort to promote active learning.
Gavin's Calculus Projects - consulting style projects for calculus students
Math Insights - A collection of short review style articles and supplementary Java applets.
Writing in Mathematics - A collection of writing projects for calculus students
Wolfram Demonstrations - A database of Mathematica applets.
Other
The DOCEAMUS series in the Notices of the AMS usually has good food for thought on the topic of mathematics education.
Teaching Time Savers was a series of articles in MAA FOCUS designed to share easy-to-implement activities for streamlining the day-to-day tasks of faculty members everywhere.