In the past week I've come up with new architectures for web-based instant messaging (Pager v2, incoming) and our whosonline functionality. I can't take full credit for either, of course, and I'm not the one doing the coding, but it's basically my design. It's pretty cool, and so far the preliminary tests are showing that the solution is probably 100x faster than the old solution that we canned, and on top of that it's more modular and much more scalable (which is related to both its modularity and its speed). Hell, it's even going to use less bandwidth, too.
A week or two ago the CTO pulled up Match.com's instant messaging client - which is basically just a really-fast auto-refreshing web browser window. They refresh every second or two. Our old web-based IM refreshed every 30 seconds or so and was still killing the boxes and very unreliable. We assumed they must be throwing huge amounts of resources at it.
Well, he did some digging, and supposedly the company that wrote it for them runs it on three webservers (and some back-end servers, I think). Or at least did initially; I'm not sure how much load those three webservers can take. At first, I didn't believe it. It had to be bullshit.
It's pretty badass.
I then used the idea I came up with for Pager, and figured out how we could apply it to Whosonline (another tool of ours that shows you on every page which users are logged into the website currently). If you've played around on the Yahoo pages you've probably seen Yahoo usernames with a little face, either asleep or smiling, next to the usernames. That's basically what our whosonline is, the difference being that as far as I know we did it first - and if my idea works, we'll be doing it better, too.