I created this plugin because the built in "distribute" tool wasn't giving enough control and the "grid" tool being very powerful requires more settings than I want to deal with every time. 

Evenly space – vertically or horizontally – selected Layers, Groups, Artboards and Symbols by a defined amount:

1) Select the elements
2) Press Command + Alt + Shift + S
3) Enter a spacing value

Enjoy!
Francesco