Voggenreiter Poster Harmonielehre

I have this both at home and in my music school.
The print quality is amazing, you can't see a single pixel, the colours are nice and unobtrusive and one glance at it's contents is enough to remind you of whatever mode or chord you just forgot how to play.
It contains a fair amount of basics without being too detailed.
Of course, you should also utilize books or other means of educating yourself about harmony, since this is not going to teach you a lot! But it serves me as a quick reminder when I'm in my workflow.