Pansies and violas can be planted in autumn and flower in spring.
Primroses and polyanthus are perennials that flower then. All these are better planted in autumn than winter though.

Helebores and Pulmnaria are very early flowering perennials, also Daphnie which is a shrub with pretty fragrant flowers.

Any of the varieties of Dicentra ( mine is " Bleeding Heart") Aside from the customary spring flowering bulbs this plant, to me, is the first true sign that my garden is wakening again and another year of gardening has started.

Many of the early bulbs can act as perennials, too. Crocus, Snowdrops, and 'February Gold' Narcissus come to mind, and there are many others available.

Leopard's Bane is a fairly tall yellow daisy which blooms in spring. You could also try wallflowers (plant in autumn) and leave them in a year or two when they will grow bushier and flower earlier.

Depends what you mean by Spring, but from March/April on to May, loads - those already mentioned, along with Iberis sempervirens, Hellebore varieties, Saponaria ocymoides, Phlox subulata, Aubretia, Arabis varieties, Brunnera, Pulmonaria, Myosotis, Doronicum, Aurinia saxatalis (previously called Alyssum saxatile); Primrose, Primula, Polyanthus, are usually quite early, along with bulbs such as snowdrops, winter aconites, crocus, Chionodoxia lucillae, Iris reticulata, Muscari, some varieties of daffodils and tulips, and in May Alliums such as Purple Sensation. All these are low growing plants, by and large, but there are spring flowering shubs as well.