I think it depends on the class. My students are high school/remedial math freshman students. The school websites I've looked at weren't too encouraging. These kids lack the type of fundamentals that you and I had mastered by the 5th or 6th grade. Their teacher's websites don't really cover that kind of material.
Now I would have loved that kind of website for my college math and science classes.
Obviously the website material needs to be tailored to the students' skill level. But I can see where a website could be useful to remedial students, by giving explanations of the same thing over and over again, using different examples, and different ways of explaining, and putting it in a format where students can review it as many times as is necessary. My Chemistry website also had textbook publisher-provided interactive features, like homework problem sets and quizzes. If it hadn't been prone to frequent technical glitches, it would have been useful. For example, it would provide "hints" on how to go about solving a problem. For remedial level students, a site programmed to correct errors in each step of multi-step problems (like long division or multiplying multi-digit numbers) might be helpful. Of course, a website is only useful to students who are motivated enough to use it, which many be a big problem with your students.