Studies have shown that students leave introductory physics courses almost universally less excited about the topic than when they came in. This article details an experiment to address this problem: a course weblog or "blog" which discusses real-world applications of physics and engages students in discussion and thinking outside of class. Students who read, commented on, and were involved with the blog maintained their initially positive attitudes towards physics in contrast to the typical deterioration in attitude seen in students who did not participate in the blog study.
However, there's one huge flaw in this research -- reading and commenting to the course blog (which was written by the instructors) was optional for the students; those who did so received extra credit. At least in my experience, usually the people doing things for extra credit are the ones who don't need the extra credit anyway; they do well in the class because they actually like the material. So the students who pay attention to the blog are probably the ones who weren't going to get disillusioned about physics no matter what.
This blog isn't a "course blog", and I don't know of any in mathematics at the introductory college level. (Or really at any other level -- Terence Tao has been posting lecture notes for his graduate course, but that's not the same thing at all.) It would be interesting to see if something like that could work, especially in a class where there are real-world applications of the material. (The "Ideas in Mathematics" course I mentioned that I'm TAing is not such a course, because of the particular preferences of the instructor; other courses offered under the same title could be.) But I suspect that one really has to make the blog an integral part of the course in order for it to have the desired effect, and even then it's not something students are used to so there's the distinct possibility that they'll dismiss it as something that was "added on" to the course.
Edited: There's a longer version in the arXiv, which I haven't read:
Gintaras Duda, Katherine Garrett. Blogging in the physics classroom: A research-based approach to shaping students' attitudes towards physics. arXiv:0708.1759v1 [physics.ed-ph]
Edited again: Upon reading the full article, it turns out that there is a small positive correlation between course grade (even excluding the extra credit available) and blog participation.