/System/Library/CoreServices, and by useful things, in this case, I mean Server Assistant. That's where it lives now, along with Directory Utility, and a few other useful things like Screen Sharing and Kerberos.
This comes up because I needed to create some AutoServerSetup files, something I hadn't yet had to do for Snow Leopard. Naturally, I go looking for Server Assistant in /Applications/Server, where it would have been under Leopard, and of course it wasn't there.
CoreServices seems to have become a dumping ground for things that fall into the "Apple would rather you didn't run these unless you really need to" apps. There's other ways to get to them, of course. You can start Server Assistant from Server Admin, and Directory Utility is now launched from the Login PrefPane. But it just seems like a pain. Directory Utility is a useful utility. Put it in the Utilities folder. Server Assistant is useful when you have servers to set up. Put it in the Server folder.