Odd angle object alignment

Question

I need to attach half of sphere to this surface, but I don't understand how do I go about aligning it.

Answer 1

1. Select desired vertex.
2. Press Shift+7 on the numpad (and maybe 5 to switch to the orthographic mode). It'll place viewport camera directly above your plane.
3. Press Shift+S and choose "Cursor to selected".
4. Add a sphere. In the operator settings tick "Align to view".