OS X Lion with various server functionality added. Most operating systems these days are the same between servers and clients, the main difference being the features and some settings tweaks, and for Windows the unlocking of various artificial limitations (such as maximum processor and memory usage).
I’ve read some speculation that Lion Server may be headed towards offering local iCloud like functionality. Which i wuld like rather than having all my stuff transfered around in the big cloud. Don’t know, I’m just downloading the desktop at the moment.