(Not for use in challenges)
I stumbled across this by accident:
1. Move your new Sim or family onto an empty lot.
2. Go into Build mode.
3. Download an unfurnished house from the Gallery.
4. Start filling the house with furniture. You'll notice that in the place of household funds you have an infinity sign. The sky's the limit.
Note: Once you start playing with the family, the cost of more furniture will be deducted from household funds the next time you go into build mode.
Of course, you can also use the freerealestate cheat, but this is handy if you have trouble getting the cheat to work or you want to put in furniture yourself.
Another discovery -- you can add things that are career rewards -- the bed from level 10 of the Painter's career, for example.