[someotheridiot]

https://rebrickable.com - A LEGO database that shows you which sets you can build from your existing collection, also includes thousands of fan-submitted designs.

I guess it would only loosely be called "passive" as I spend a lot of time on it. But, I can take vacations and it continues to earn... for a while :)

[joebaf]

I've seen it some time ago and this site is great! it really brings value to users, so I am not surprised it's growing.

[Hydraulix989]

I saw that site a couple weeks back. I have a giant tub of Legos in my attic. Very cool!