Been having a bit of a play with MC Server this evening (www.mc-server.org, I think).
It's a custom server written in C++, and it seems reet fast. Running it on my laptop and I can play happily with view distance set to far when connected to the server. WHen playing single player I can't really go above 'short' view distance without it becoming unplayable.
It has plugin support (teleport, items, lists etc) and a better file format that's more efficient and doesn't spawn 100K files for a level. Also allows for a web-interface for server admin stuff, which'd be good if the server needs tweaking/restarting when I'm not about.
The downside? It doesn't really work yet beyond letting you connect to a world and walk around. :( Also, the guy that writes it seems to be a bit of a cock judging by their forums. There's also no useful information/documentation (less than the offical server :S) and no indication of when it was last updated. The file format is also incompattable with the existing world and there's no import/conversion tool.
So basically of no use to us, however it does hopefully point to what will be available in the (hopefully not too distant) future. If only Notch would move the official server to a proper language we could have Mobs and everything on :(