http://g.cn/

It re-writes to http://www.google.cn/webhp?source=g_cn

But that is the shortest one that I know of.

I think g.cn too, because there aren’t any 1-letter Top Level Domains as far as I’m aware, and you can’t get a shorten name than a one-character name ;-)

(Though there are probably more four-character domains, it’s just that g.cn was in the news recently ;-)

LOL! Any domain with one letter/number in its name.