MeshCast
Generate printable molds and maker projects in your browser
Tag shelf
STL tools and resources
A focused list of resources connected to STL. Useful when you already know the kind of problem you are trying to solve and want fewer tabs open.
BumpMesh
Add displacement textures to 3D models locally in your browser
Formware Online STL Repair
Free online STL file repair service
Thingiverse
Probably the largest free 3D model repository. Owned by UltiMaker.