Nothing

Paper and pencils.

I'm using floorplanner.com not awesome but not awful for free...

Might be overkill, but it's pretty good - sketchup.com/

all the designers on HGTV use SketchUp, weirdly.

I think there is actual HGTV branded home planning software too.

i made a pretty nice sketchup model of our house in an hour or two. the hardest part is getting room sizes after move-in

try to do that part before your crap's in the way. or find a realtor that has a sonar measuring tape.

Sketchup was the first thing that came to mind and the Internet seems to like homeplanner as well, but there's srsly like 6,000 home planning softwares out there!

Sketchup would be my recommendation.