Here is a little walkthrough on exporting your Houses and Lots so they can be uploaded, duplicated, sent to a friend. Advanced computer users will already have figured this out - but here, on request, is a rock-bottom tutorial for non-geeks.
For a Mac, it's just a little different.
When you left click on the house, a window comes up with the family and/or the info on the lot. On the left side of that window there's a "show family relationships" button and an "info" button. Click the info button and more detail comes up, including a window for the family bio as well as the arrow with the asterisk at the top for exporting custom content (it actually says "Package Lot" when you hover over it).
Sims 2 game files are stored, in OS X, in the user's Documents folder under EA Games.
Notes for Mac were contributed by AuKestrel - thank you!
Sims 1 users in particular can find this confusing as the directory structure is a little different in Sims 2. There's no need in Sims 2 to navigate to the Neighbourhoods\Houses folder of your game to find a particular house. Walk through the process with me - it is very easy.
When you have built your house, go to the Neighbourhood Screen.
You will be offered the choice of saving your lot - usually the answer to this will be Yes.
Locate the house you want to export and mouse over it. Single left-click of your mouse will bring up something like this:
Notice how the white oval appears to come from your house?
The icon 
is the icon that you need to
look for - it means export a piece of custom content. Click that.
This gives you two options.
I am going to use the second option, as it allows me to save the lot to a file I can then put on this website. Tick the box and the Accept button.
The game will pause a moment and then you should see a blue window something like this:
Now look into that directory of your computer. Using 
My Computer,
(icon on your desktop in Windows - sorry, Mac users), navigate to
(Pitfall to avoid - unlike Sims 1, Sims 2 uses a folder in My Documents. So it's no good on my computer, for example, looking in my Sims drive! Later I will show how to move your My Documents folder off your C drive if that is getting too cluttered.)
So it's My Documents\EA Games\Sims 2\Packaged Lots. There is your house file. You can now share it with other users via a website or Yahoo Group; or if you want multiple copies, or even a whole housing estate of this type of house, first shut down your Sims 2 game.
Now double-click on the file that is still open in My Computer - the built-in Sims Package Installer will install the house lot again in your game each time you double-click it.
When you have another, or enough copies, it is quite safe to delete the file from this folder - the original house is still in your game. Each time you double-click the file, a copy of the house will appear in your Houses and Lots Bin. Any other user with Sims 2 installed will be able to install this house exactly as you made it, from this file.