There is an iris companion plants thread on here somewhere - if you look closely you might find more than one.
Other than roses and peonies, I grow lots of spring blooming bulbs, alliums, pincushion flowers, sun-loving heucheras, columbines, mountain phlox (Phlox subulata), small Japanese maples, sempervivum, blue fescue and much, much more. Small blue conifers also go particularly well with the fans on the irises. As long as you do not shade out the irises, the list is almost endless.
