The Sims 1: I don't know what it was, but It was a hacked object resembling a fire alarm that you could turn "on", and visiting Sims would act like they lived in the lot. They'd use the beds, cook, whatever. Unfortunately they'd skip work and be fired, but it meant you could have visitors who HUNG AROUND and didn't just leave after a few minutes.
Runner up: a safe that your Sims could break into for varying amounts. I always stuck them in my Downtown lots so my criminal sims had a side activity waiting for them.
The Sims 2:
The Mind Control Mirror that let me give orders to visiting sims.
Runner up: ACR, which let me set sexual orientation rates for the neighborhood, and allowed for amusing romantic spotenanity if I needed drama.
