Straighten/align a selection of faces to another face using the pivot alignement

[bloox64]

I recomend you using the mesh align plus addon. Also the BS Modify pivot addon could be useful at some points for you

Mesh Align Plus

ADDON SUMMARY (Edited to include extra info/details) Mesh Align Plus helps you move things around, precisely: arrange objects in your scene, align mesh parts to each other while you’re modeling, and create complex custom transformations using measurements from your models. In other words, Mesh Align Plus is designed to provide precision modeling capabilities, especially for hard surface modelers, mechanical, architectural and CAD/CAM users. Some real world examples of things you can do with t…

BS Modify Pivot Addon Release!

Hi guys. I want to present our first addon for Blender. In this case, since we come from 3dsmax and Maya, we missed a lot the ability to change the objects pivot orientation and I saw a lot of people here in the forums asking for the same thing, some people suggested to use the Hook modifer, but we wanted something simpler, similar to the ¨Affect Pivot Only¨ button present in 3dsmax… so we did it Here you have a video explaining it: And here is the direct download link to the…

Edit: I relaized that wasnt’t exactly what you asked, but those tools are quite useful, as @Way has said I would also use the active vertex, much quicker than using the 3d cursor.

If you don’t need keeping some vertices of the face you want to align with, you can also align the viewport to face with shift+7 and then with the looptools addon flatten the selected faces.