This isn't directly prop related, but since "Call of Cthulhu" gaming is such a huge part of Lovecraftian fandom I wanted to post about it.
There are RPG mapping programs on the market capable of handling everything from a single room in a dungeon to entire stellar clusters, but if you're looking for something that's unbelievably fast and easy to use I think you might like Floorplanner.com. Granted, gamers aren't it's intended audience. The free, flash-based program is intended to help people design the layout of homes and landscaping. It just so happens that the smooth, drag and drop interface they've come up with is also ideal for banging out the kind of floorplans that are ideal for gaming.
In about five minutes I was able to put together this quick "Temple of Dagon" map:
Not only is it a snap to layout a room, building, or even a starship deckplan, but Floorplanner can take your 2D plan and render it in 3d with the click of a button:
Mind you, this was something I banged out within minutes of stumbling across the site. There are so many options and objects available that you could spend hours creating an entire house down to the smallest detail. Which isn't all that suprising since that's exactly what the site is intended for.