I've been wanting to get into java for a while now. Only recently did I gain enough courage to start modding minecraft.
This mod is a clientside mod that allows you to place in-game references to make precision building easier.
(more images: http://imgur.com/a/LJZep
Currently, the keys are not yet configurable. The key to open the settings GUI is G (for Guide) and the key to place a reference is R (for ruler).
This is a very early dev release (0.1.0 didn't even have a GUI), so the capabilities are very limited.
In the GUI (which is still buggy), choosing a type from the left sidebar will bring up their parameter boxes. Currently, there are only two: crosses and flat circles, each taking a single number as their "radius". To "confirm" the settings, simply press Esc then use R to place the guide. Placed guides will be listed from top-down on the right sidebar; choosing one and clicking "Delete Pattern" will destroy it.
In the future, there will be far more types (catenaries, polygons, bezier curves) and each type will be more customizable (rotation, offset, colors).
Here's the download link for minecraft 1.12 (requires Forge):