Lots of possible choices! Heucheras are great container plants, often fair better in containers than in the ground and offer a very wide range of foliage color. Smaller ornamental grasses are also a good choice - many of the Carex species will work, also hakonechloa, fescues, black mondo and liriope (although the last two not technically grasses). Wintergreen offers a spreading habit, flowers, berries and winter color. Spreading bellflower (Dickson's Gold, Birch's Hybrid), hardy geraniums, sunroses (Helianthemum) and lots of the low rockery plants will work, at least for some seasonal color. Heaths and heathers also. For more trailing type plants, look to perennial groundcovers - vinca, lysimachia or variegated ivies are good choices (keep the ivy in a container, please!)

Since most perennials tend to have a rather limited bloom season compared to annuals, I prefer to focus on foliage attributes and color rather than flowers. Some of the new hybrid hellebores ('Ivory Prince', 'Silvermoon', etc.) offer both gorgeous, longlasting early season flowers as well as extremely attractive evergreen foliage. Bergenia, hostas, ferns, small hebes and sedums are all possibilities as well. Select for specific sun/shade situations.

Gardengal has lots of good suggestions. If you were hoping for more of a floral display, you could tuck in an annual or two amongst the perennials in some/all the pots. You'd avoid the cost of filling all the pots with annuals every year, but get a little extra color & floral interest. Each year you could vary the annuals for a change in appearance.

Diascia is the answer! Bright colours, very long blooming season, comes back every year (I have had (and divided) some of mine for 5 years now) trailing and almost evergreen..... cut back dead stuff in the spring. I have some of it in all my pots, and it will grow high ( around a potted rose) maybe up to 2 1/2 feet with its support) It is just great.... coral, pink, or dk. pink.