Like toying around with floorplans? Fancy yourself an interior designer? Want to play like the big boys but on a cardboard box budget?
Autodesk Homestyler is available free on multiple devices: web, iOS, and Android. I have played around with Autodesk Homestyler on Chrome (the web app is a glorified bookmark to http://www.homestyler.com/designer) and found it to be quite the time waster. Fun, time waster. Maybe not exactly wasting time, but definitely finding myself addicted to it and not knowing where the hours have gone. (On a side note, I really like the color pallet I picked out to go with the new hardwood floors in our living room redo.)
Homestyler is not a full-blown architectural design tool, but its intuitive user interface and built-in design elements make concepting an enjoyable experience.