Cura is a slicer that was originally developed by David Braam, working for Ultimaker. The software is has been released under the open source Affero General Public License version 3 and can be downloaded here.
The Cura slicer is used for many printers and can handle almost any standard 3d file, like STL, OBJ, etc. It’s a pretty fast slicer that has a lot of settings and sometimes the slicer can be found in online sources, like AstroPrint and OctoPrint. It has less options then Simplify3D, but there are no costs to use it and also it comes shipped with a whole lot of 3D printers these days.