Random tiles / editor?

I'm trying to figure out the easiest way to create a random level (much like the dungeon creator. That part is easy) but with the ability to have the mouse "snap" to each piece

I'm looking to create somewhat of a level editor, where I can place different blocks down. It doesn't seem too difficult, but the in game snapping idea has me baffled. I'm sure its very simple to do, but the method escapes me

This is what I use for grid snapping:

x=int(x/16)*16

y=int(y/16)*16

You can change 16 to whatever grid size you want.

Perfect my friend, perfect

Works like a charm. Thanks