This SE site is dedicated to GIS, but there are related topics which might not be completely in topic here and deserve their own sites. Let's help them to take off the ground!
Here are some of them:
CAD/CAM software - there are some areas where GIS and CAD meets, so could interrests of their users.
Geography - another proposal where cross-interrest is obvious.
Geoscience - similar to the proposal above.
Archaeology - one of fields where GIS is applied. It's not very important from GIS perspective, but I know that at least one archaeologist is or at least was here, and I hope that me and him are not the only people interrested in GIS applications in archaeology here.
This post is a community wiki - feel free to add more examples of related Area51 proposals.