Description – This is pretty much the standard for firmware configuration and deployment. When you need to get Marlin to your Ramps board it’s the only way to go.
KISS Slicer 
Description – Really fast single material 3D slicer. This code is free but it is not open source. I’ve found for simple and fast print G-Code generation this is a great tool.
Description – Great open source tool and it’s getting faster every release. I’f you are playing with multi-material 3D printing it’s one of the best open source solutions.
Description – For strait forward model work this is the tool. It’s limitation is its all command line with no GUI for development but if you use a editor like Notepad++ it’s great. It’s also really good for parameter driven model design.
Description – Best tool I’ve found to integrate with OpenSCad. Lot’s of plugins for OpenSCad.
Description – This is a great 3D printer controller. It’s not complicated and it’s been working for me on both my printer.
Description – Currently best and largest free 3D printing model repository I’ve found.
Description – Great little 3D printer controller card to provide control an access from web browser.
OctoPrint 
Description – 3D controller browser based software. Installs available for Raspberry PI
RepRap 
Description – The only place to go if you want to build your own 3D printer and a great place for general 3D info.
fortlaud3d 