You can do 2d floor plans in SketchUp pretty easily.
Just create some construction lines and copy them at whatever 'grid' you want... but why not just point the line in the direction you want and use the built in directional locks to make it draw square corners and straight lines?


You could try the free version of DoubleCAD. Not sure if there is a built in grid - but if it is similar to AutoCAD LT you should be able to set one up.




You could draw by hand - you don't even need to figure out how to do it in a spreadsheet... just graph paper, ruler, etc.



Why is a 'grid based' program easier?