I love Duotrope. Their search engine helped me find great short story markets like Penumbra and Cast of Wonders. Duotrope switched to a subscription model in January – a choice that I don’t begrudge them.
If you are looking for a free alternative to Duotrope, though, you should check out The Submission Grinder. You can search for markets based on genre, word count, and pay scale. It offers many of the same features as Duotrope and promises to always be free.