What zone are you in?

My favorite all season bloomer is Delosperma cooperi. It has larger blooms than the creeping phlox and the foliage doesn't get the ugly brown patches. It blooms from April to frost. Even cuttings of it can bloom within days.

Primrose?

Lamium. I've got Lamium Purple Dragon and it flowers on and off all year.

echinaceamaniac, you forget to mention that the Delosperma is also evergreen! I'm crazy about it, too.

a great many of the hardy geraniums will perform that sort of service. Prunella (selfheal) might also work and you might take a look at the smaller campanulas such as C.portenschlagiana or C.pulla. There is a symphyandra with an unpronounceable (and unspellable) name - sounds like zanzegura and failing that, the little vinca minor - 'Gertrude Jekyll' is well behaved and mannerly.
For sun, the sedums and helianthemums are good value.

Forgot my zone... recently upgraded to Z6 near Cleveland OH

How about Veronica prostrata 'Georgia Blue'? I put this in last year from a 3 inch pot and it now covers about 1 square foot of space. It's been in full bloom for a couple of weeks, but I'm not sure how long it will bloom. From what I've read, it is said to be a rebloomer. In any case it's only a few inches high, the foliage is lovely and the flowers simply cover the plant.

Kevin

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I made an error. That plant didn't come in a 3 inch pot, it was part of a 6-pack from Bluestone, so it was really tiny last year.

Kevin

Kevin, good call with the veronica Georgia Blue. As a matter of fact, I have a swath of blue phlox around a bed of mixed daffodils and grape hyacinths. At first it was gorgeous, but I can't seem to keep the phlox thriving. I have finally decided to give up on it. A few feet away, in another bed, is a large clump of veronica Georgia Blue, and one day while walking out to my mailbox, I had an epiphany! I saw the veronica, simply smothered in those little blooms, and thought: Switch out the blue phlox for the veronica!

So that's my plan. I just need to get (lots) more veronica. Ibanez, it's not an all-season bloomer, but no perennial really is. I would say it blooms for a few weeks, though, and after peak bloom there are still blooms here and there for a bit.

Dee

Give it time and you will have lots and lots of veronica. I rip up big patches each year to keep it in check. I dump wheelbarrows of that stuff. All from 2 small pots.

Veronica 'Blue Reflections' and 'Waterperry' are nice too. But they are not in bloom for all that long. I don't know of anything that is. Maybe the iceplant, but it doesn't grow well in my clay soil. If you are looking for full sun? I have Veronica in both full sun and part sun and I was very surprised how quickly the full sun Veronica filled in. The part sun has stayed in place for a few years