Our department is currently revamping our first-year courses in mathematics, which are huge classes (about 500+ students) that are mostly students who will continue on to Engineering.
The existing teaching methods (largely, "lemma, proof, corollary, application, lemma, proof, corollary, application, rinse and repeat") do not properly accommodate the widely varying students.
I am interested in finding out about alternative, innovative or just interesting ideas for teaching mathematics - no matter how way out they may seem. Preferably something backed up by educational research, but any ideas are welcome.
Edit: a colleague pointed out that I should mention that currently we lecture to the 500 students in groups of around 200-240, not all 500 at once.
You could try a problem-based approach. Just in case you don't read the rest of this answer, let me offer a couple of links:
Phillips Exeter's math curriculum is entirely problem-based, up through multivariable calculus. Exeter offers their curriculum to the public for free, just click on "Teaching Resources" and download the problem sets. For a first-year calculus course, I recommend the Math 4 problem set. Yes, this course is aimed at high school seniors, but they're Exeter seniors---they're basically college freshmen.
If that's not rigorous enough, try the University of Chicago's Honors Calculus sequence (the link takes you to one of the instructors' course site). Read the syllabus first, then download the sheets.
In problem-based (or "inquiry-based") learning, rather than saying "And now, our next theorem is Basson's Irrelevance Theorem; here's its statement and here's its proof," you instead pose a series of problems that the students must work on and prepare before class. Perhaps the problems are similar to the ones that Basson was trying to solve when he developed his famous Irrelevance Theorem; perhaps they are special cases of the Irrelevance Theorem, leading up to a general case; perhaps they simply guide the students towards some concept.
In any case, in the next class, students are asked to share their solutions with the group. They may not have a complete solution, and this is ok. They may have only part of a solution, in which case they should say "I got this far, now what?" This motivates a discussion of the merits of the various approaches, guided by the instructor ("Thank you for proposing that approach, but have you considered this wrinkle? How does that affect your solution? Is there an alternative approach that takes this into account?"). By the end of the discussion, the group has arrived at a solution and can move on to the next problem.
Pros
A problem-based approach has several advantages:
The theme here is that students learn math by doing math. The class time is mostly discussion-based, and the students are the most active participants in the discussion. The instructor is a guide or facilitator, to help students see flaws in their reasoning, to bring up potential issues or edge cases that might not occur to them, etc. On a really excellent day, the instructor practically doesn't even have to be there.
Cons
I'll end this by saying that in my opinion, the pros far outweigh the cons. As far as the last con goes, if you're considering a radical revamping of your curriculum anyway, you can probably get the requisite buy-in from your instructors. Best of luck!