And there is an effort underway to build a web-based Topology Course, at what appears to be an undergraduate level. It's probably worth checking out, just to see what they're up to?
There is a fairly wide ranging discussion of General Topology (i.e., point-set topology), which is part of a larger group of pages on the various divisions of mathematics.
More generally, there is the rather entertaining Interactive Mathematics Miscellany page. My favorite little tidbit so far has been "[By the extreme value theorem,] for any object there is a distance at which it looks its best." (It took me a little while to figure out why that was actually true.)