maryrobinette has a post about nonpaying markets, and we got into a discussion of whether it's ever worthwhile to give away a piece of writing. I allowed as to how some magazines have an instant niche and are worth supporting, given that I am an activist as well as a professional wordsmith, and there are some topics I customarily work to advance.
I think that depends on the market. Zippy's New Science Fiction? Not much of a niche. Alien Queers? Alternative Histories of Color? Form Poetry of the World? Crowdfunding Creatives R Us? Bilingual Conlang Fantasies? Those are instant niches and I would be all over them.
What are some magazine titles that would be so enthralling for you that you'd at least consider giving them some of your work for free?