How to get (almost) perfect texture rotations for brush faces in Hammer 3.5 using Blender
This tutorial applies to the Hammer Editor 3.5
UPDATE: I have removed the old tutorial since it was totally dispensable. Read the comments by 3zzzTyle for further information about this topic.
Aligning Textures on edges works by selecting the neighboring face and then pressing ALT while applying the selected texture to faces. In almost every case this will apply the correct rotation, scale and shift.
In cases where this doesn't produce the expected result, try switching the texture alignment of the face to "Face" instead of "World" and it might work perfectly after all.