Since there's an abundance of short oneshots that have been scanlated and I've enjoyed many of them, I'll focus mainly on collections of short stories.
I'd highly recommend Ranman. Although the series is listed as having four chapters in one volume, each of the "chapters" is composed of numerous unrelated oneshots, with around thirty or so in total, and only one of the stories even reaches 20 pages.
With a couple of exceptions (chapters 6 and 11), the oneshots in Shiroi Kumo are short, and they're all well worth reading.
Expanding on Tiffyyii's recommendation, all the oneshots from Le théâtre de A scanlated so far are short, interesting stories.
If you enjoyed New Century Godslayer, you should check out Kago Shintaro's Genesis, from Kasutoro Shiki - it has a similar premise. Kago's done several volumes of oneshots, so if you enjoy Genesis, you should check out his other works as well.