Duotrope is a free site; they ask for donations to help keep it that way. I have happily donated in the past and plan to do so again in the future. You do have to register to access some of the features of the site, but since it's free to try, though, you can get a good feel for its value to you before making any financial committment.
And now, three benefits of using Duotrope:
- Market research and submission tracking. This is what I'd call the "core competency" of Duotrope. You can use Duotrope to research markets which might be interested in publishing things which you have written. You can also track your submissions on the site; this allows you to easily avoid submitting two stories at the same time to a market which doesn't allow for this, submitting the same poem a market has already rejected to them again, etc. Now, I do suggest that people have some kind of backup of this submission tracking data external to Duotrope, just in case something should happen and the service should be temporarily or permanently unavailable. But as a day-to-day resource, I find it very easy to use.
- Keeping track of upcoming deadlines. Duotrope has a Theme and Deadline Calendar feature which can help you keep track not just of themed deadlines like those I write about every Thursday but also when submission windows are closing for other publications.
- Spotting signs that a market might be having difficulty. Admittedly, this one involves making some educated guesses. But there's a lot of information around the response time statistics of markets which can be viewed on Duotrope. Often, before I submit to an unfamiliar market, I will look at the "Response Times" section of the market's Duotrope page and also click on the link on that market's page titled "View report of recent responses from this market." Between those two sets of data, I can see if it looks like the market has been doing a poor job of responding to recent submissions and that does sometimes influence my decision of which markets to prioritize.