Using Half life game to design a house neat idea

Here is a neat idea, rather than investing in hundreds of dollars in 3d system why not use the half life game engine to do your 3d modeling. Some guy built a house in the game and you can go through it as in a first person shooter which is really great. Hope in future many people use this tech to do their 3d visualizing and give a better idea to their clients.  What other games would be better to do such things. GTA for city buildings and Far Cry for beach house, Star war games for Space colony modeling etc. Any more idea...