From Adam Frisby's blog, news of improvements to OpenSim. (Again, it's news to me, which shows that I've not been keeping up.)
I'm highly impressed--they've made significant improvements to how OpenSim moves data across the network. We're talking a 71% reduction in packets transmitted with 100 avatars in a region, and 29% drop in CPU requirements... oh, wait, that's just for one change. Another change to allow faster garbage collection of packets cuts CPU requirements by 40%, again with 100 avatars. Let's see, (1 - .29) * (1 - .4) is a bit under .43, so the two improvements together cut CPU requirements by about 57%.