I started out with Slic3r but then switched to Cura. Now I always use both and compare the results. I made a droplet tool for OS X so that I can slice several files at once in a batch. This saves a lot of time and effort, since I have to re-slice a bunch of models (Prusa i3 parts mostly) whenever I switch to a new roll of filament. I have heard good things about Skeinforge, but it's very complex and intimidating. SFACT is a simplified slicer, as is KISSlicer, but I have not used those. There's actually a commercial slicer, Simplify3D that looks really really impressive and fast. Then there's AstroPrint, which sends the STL file to the web for slicing, then sends it to your printer wirelessly. Expect more web-based services like that, all-in-one solutions like Makerbot's software, and new things from Ultimaker soon.