Assuming you've already read Alicorn's, of course? They took me much longer to get around to than I can possibly justify.
http://alicorn.elcenia.com/stories/stories.shtmlMost of the shorts I read are in dead-tree format, sadly; I don't know where you might find good sources online. If you want just a handy source of online free (for non-commercial redistribution, even) fiction, some of which is shorts, take a look at
http://baencd.thefifthimperium.com. I can't vouch for the quality of all of it (it all got published commercially, but that doesn't mean much; some of it is decidedly inferior to, say, Luminosity) but there's a decent range of authors and writing styles and plenty of it that I greatly enjoy (to the extent that an offline copy sufficed to keep me in reading material for some ten weeks, a rather impressive achievement). SF/Fantasy, in case the "Baen" in the link hadn't tipped you off.
I've lately been reading an awful lot of LessWrong (came to it later than I'd have liked), including a good bit of stuff that Eliezer Yudkowsky wrote (the fiction, most of which is short, is linked here:
http://yudkowsky.net/other/fiction/).