A 'perennial boarder.' Is that the guest who won't leave? :)

Really depends on what 'look' you have going on in the rest of the garden. Low mixed groundcover type plants are nice. Geranium Biokovo and G. Bevan's variety are two good ones for the front of the border that can probably take full sun. I use these a lot in different light conditions. Low growing sedums will take blazing sun and drought. Sedum Angelina is a newer one I like. Nepeta 'Walkers Low' is nice under roses. It's not 'low' but suitable for front of border. And there's always Liriope, shorter grasses. Daylilies are nice mixed in the front as the leaves curve.

I'm very fond of candytuft (Iberis sempervirens) and crocus are nice in the front of a border or if you want something that blooms really early how about snowdrops.