Whenever a chunk is generated by the mapgen, anyone within a given radius (500 blocks by default) will be awarded a certain number of points. After they collect enough, an inspiration is added to their inventory.
Building is defined as placing blocks. More decorative blocks have higher point rewards. It's quite easy to game the system by repeatedly placing and digging the same blocks, or just dumping loads of dirt everywhere. I'm not all that concerned about making it more particular at the moment.
Note: only the default game blocks are awarded particular values. Any nodes added by another mod will default to double the value of dirt.
This is a work in progress.
The source is available on github.
Code: LGPL2, textures: GFDL
Mod dependencies: default
Edit: I'm ceasing work on this.