[cryzinger]

Not an official site, but Rebrickable has a section for "alternate builds" you can use to repurpose an existing set: https://rebrickable.com/sets/alternates/

[ygra]

Rebrickable is awesome. One of my children loves building with instructions and will do relatively little free building with Lego, and Rebrickable makes it very easy to find alternative models (often with questionable stability, though) that can be built from the same set. It also features a search of alternative models you can build with all the sets (and bricks) you have.

[miningape]

This is really cool