Most people are aware that you can pull a horizontal or vertical guide out of the document ruler in Adobe Illustrator. But I suspect many users are unaware that you can then rotate that guide to any angle you wish.

Drag a guide out from the ruler. Now make sure your guides aren’t locked by unticking Lock Guides in the View>Guides menu. Select the guide (you can click and drag over the guide to easily select it) so it is active. Now select the Rotate tool (or simply hit the R key) and rotate the guide to your desired angle. Once you’re finished, you can re-lock the guide to keep from accidentally moving it.