I currently have all of my tanks in my upstairs livingroom, and I'm really trying to find room/support for more in that room because I like to have them all in one place to look at them all, for ease at feeding times and water changes, and because it's the best room to support the power needs and easiest to clean up if there are spills (no carpet!). Being an upstairs room, however, I'm having trouble justifying more tanks, especially large ones, due to weight, though space isn't really an issue yet, unless I do get the big tanks I'd love to have. Most of my tanks are on built-in stands like built-in bookshelves/wall units, my 65 resides almost inside the fireplace, and my small tanks are all over the place on end tables, side bars, hutch, etc. If I were to get more tanks (50 gallons or less at this point, since my 100+ gallon dream is to expensive for me right now), I'd get metal stands that can support two tanks.
I'm lucky that we don't really use the livingroom for anything, though. We have a seperate TV room and dining room, so the livingroom has pretty much been turned into a fish room that happens to have lots of books and some couches in it, too. Makes for a very pleasent, relaxing reading area, though.