Ortho offers a collection of tools that allows you to move, rotate, scale, snap and align selections of a mesh relative to a user-defined reference plane.
Working relative to a reference plane can greatly simplify the positioning of mesh parts and can help clean up distorted meshes. Blender already offers several tools to transform and snap mesh parts but they work in the context of predefined coordinates, which makes it difficult to position or align mesh parts in meshes that are transformed with respect to their local coordinates or in situations where orthogonal coordinates are not sufficient, for example when positioning a window inside a slanted roof.
Ortho offers a simple and interactive way to define a plane that fits a selection of vertices in a mesh and offers a set of tools that operate with respect to this reference plane. You can, for example, align and snap a selection to the reference plane or move this selection along its normal or its surface. Scaling is also an option, offering ways to rectify slightly distorted meshes even in situations where such a distorted plane is not aligned with any axis and scaling along individual normals with Alt-S gives strange results.