Grid for a level Editor

I am implementing a Level Editor for my engine and I need an help to implement a grid like the one that you can see in the Q3Radiant or unrealED but I don''t have any idea on how to work on it. I wanna have zoom with correct tesselation of the grid and snap the mouse to the points of the grid. I have already implemented the picking function so I have the function to transform the video coordinate of the mouse in the coordinate of world space. So if you have an idea please let me know. Thanks