Fantasy and Science Fiction has a really interesting post about fee online spec fiction. Actually, the best part is the comments to the post, which should be of particular interest to authors trying to promote their work.
Just glancing through the comments, one theme becomes very apparent: if you want to sell your work, give it away for free. Huh? you may say. But just look at all the readers who say, in essence "If I read a short story by an author I like, then I will go out any buy that author's novel(s), anthology, etc." Further, the vast majority of the people would not have discovered this author if they had to pay for his or her work initially. In other words, free online fiction is probably a better source for promoting an author than print mags or subscription based e-zines. Believe it or not, you may get more bang for your buck if you get a story published in Electric Spec than if you get publish in a print magazine.
One sour note--most of the readers admitted that while they were happy to support the authors, they were unlikely to support the publishers (donations, payments, etc). What many readers miss is that one is directly related to the other, at least in the case of free online zines. For example, any support given to Electric Spec would go to things that would benefit the authors, like higher author payments and the nuts and blots of keeping the magazine alive. If the free online venues go away, that's one less avenue for author promotion.
The bottom line is that free online zines like Electric Spec are beneficial to readers and authors alike. The missing piece seems to be supporting editors and publishers.