The debug grid module allows you to put a background grid on any element which can help you line elements up vertically and horizontally with eachother. Sometimes across large amounts of whitespace it can be tough to see if things are aligned. The background grid comes in both 8 and 16px columns.
The debug module is commented out at the bottom of the src/tachyons.css file. If you uncomment it a 1px outline will be placed on every element on the page. Each element has a different color outline. The purpose of this is to help distinguish elements that are close together by giving them different colors. This has been helpful at times when the outlines are overlapping by 1 or 2 pixels and it's hard to tell where one element ends and the other begins.