Another vote for
Darktable. It's pretty decent so far, it does have quite a bit of a learning curve but it's not too bad. The community is extremely active and the development is also just as active. One thing I do not care for, is the interface. It's wicked dark and you're limited by how much you can lighten it. Some folks enjoy the dark UI but I struggle with it during the day time. At night, it's not such a big deal.
If Darktable's learning curve is too much, you might try
RawTherapee (
http://www.rawtherapee.com/) They are quite active and just like Darktable, it's completely free to use. It does have a bit of a learning curve but it feels more like Aperture or lightroom. The interface is also customizable and you can theme it / arrange the panels to your liking.
LightZone is another pretty decent Aperture alternative. You do have to register on their website to download it, but it's free to use. They aim to be more of a Lightroom alternative than anything. Their focus seems to be geared more toward editing, versus cataloging, but it's still worth giving it a try. ( lightzoneproject.org )
Aftershot Pro (corel) and
Phase One's Capture One are two great applications. However, they are not free. Both offer 30-day trials though. Over the two, I prefer Capture One a wee-bit more than Aftershot, but I would say, give both a try.
Hopefully one of these days we'll be graced with an amazing Lightroom alternative. Photos makes an ok iPhoto replacement, but it lacks some features that make it suitable for pro-grade work (IMO). Hopefully someone like Serif will step up and offer an alternative. Their Affinity Photo and Affinity Designer applications are stupendous and they are a step in the right direction for getting away from Adobe. I would love to see Serif come out with an Affinity "library" application. Anyway..
Hopefully some of those work out for you.