Your size restrictions make it difficult, but if you look into specific cultivars I think you could match up a Melampodium, Lantana (perennial, but used as an annual), maybe a Celosia hybrid, perhaps Calabrachoa. All can be encouraged to bloom all summer long.

The osteospermum hybrids are quite lovely, are only about a foot tall, and bloom all summer long. They're definitely a full-sun plant. Look for "Yellow Symphony" in the Proven Winners brand--most big garden stores carry it. I grew an orange osteospermum hybrid last year and loved it. Low maintenance, looks great.

Here is a link that might be useful: osteospermum Lemon Symphony

Cindy - there are also yellow Cosmos (Cosmic Yellow) that would suit your requirements, and are heat lovers. Osteos are pretty, but will NOT do well in your SC summer! I second the motion for Melampodium. Am racking my brain for yellow plants I have seen at the Park trials in Greenwood that look exceptional in the summer...Calendula?

I think that in her location, Calendula would be more of a fall plant. It's commonly called 'winter marigold'.

Hey, how could we forget Zinnia angustifolia? These little cuties come in white, orange, and yellow. They are a FAVORITE in our gardens and containers. Absolutely terrific in the hot, humid, summer sun. Zero disease problems for me, even in clay soils.

The attached image makes them look bigger than they are. The flowers are actually quite small, completely covering pretty green mounds of foliage.

Here is a link that might be useful: {{gwi:8784}}

Thank you all for your great ideas. Rhizo, I loved the zinnia Orange Perfusion I had last year - very reliable in the heat - didn't know it came in yellow - I'll check it out. Also like the Yellow Cosmos idea - have never grown it before, and have never heard of melampodium before. All great ideas. Thanks so much!

I love the new Stella Supreme daylily. The color is the exact shade of pastel yellow that some of my pansies were and they are supposed to bloom better and longer than the regular stellas! I bought several at Lowes. My regular stellas bloom longer than some of my annuals.

I love my Melampodiums! They are quite tough and keep their moundy shape while showing off all summer. Here is a photo, credit to vaes.org (not my garden), of Melampodium.
{{gwi:8785}}

Lantana is also a tough show-stopper but it's more spready than moundy.
Photo from hotgardens.net
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I planted Gazania this year. This isn't an annual, but rather a perennial in some zones. I found some of these at Wal-Mart, but you can also order them online if you know of a reputable dealer.

Gazania Sun Gold

Gazania Trailing Yellow

Of course, if you want an annual, then I've been of no help at all! HA!

Beverly

Wow Dicentra - fantastic picture. Looks like it's perfect for my spot. And Bumblebeez, I'm going to look for the Stella Supreme - I've never seen it. I have some of the regular Stellas. Thanks.

And the great thing about melanpodium is that once you have them, you always have them. They self sow like crazy!

I don't know if there is an annual called a yellow supertunia. I know there is one called Citrus which has yellow inside and creamy white on the outer part of the flower. I would like to get a brighter shade of yellow rather than the one Citrus mentioned above. Any ideas

Okay. Let me bring you back to zinnias. But, this time, you need to try Zinnia Zahara Yellow. It is essentially the same size and form as Profusions, but with significant differences. The color is a cool yellow, as opposed to a warm yellow (that always has undertones of gold and/or orange). With any more green, the Zahara would be chartreuse, but it is not, it is a beautiful cool yellow. And it absolutely blooms all the way until frost without let up.

I grew Yellow Profusions last year. They were taller than Zahara. Not much. Just enough to flop. yuck. I cut them back and they came right back out and flopped again. Double yuck. And, their foliage developed ugly brown spots late in the season. (And, you're OUT!.... Sorry:)

The Zaharas don't flop, mound neatly near the ground, and the foliage stays pristine. (given good soil, adequate moisture, etc.) Both Profusions and Zaharas hide their dead blooms under fresh foliage, which is a huge plus: no deadheading.

It's difficult to find Zahara Zinnia transplants in my area. So, I start them from seed. I can sow the seed today and they're ready to put in the ground in three weeks. (Grown outside on my deck.) With fertilizer, they're blooming in two more weeks. With the length of our growing season, there's still plenty of time for enjoyment. I grow several flats of them each year, so I order my seed from Hazzards.

With regard to Black eyed susans, (Rudbeckias), I have grown every kind I have ever been able to find. I have sworn off all but Goldsturm, and that I grow only because they seed ETERNALLY. I have never grown a Rudbeckia IN MY CLIMATE that blooms for more than a month. Indian Summer blooms once (spectacularly) and that's it. Herbstonne blooms once, spectacularly, and once more, pathetically, and that's it. The little wildling, whose latin name escapes me, blooms on and off again, but the plant has no substance. It's better as filler, and seeds EVERYWHERE.

Coreopsis blooom once, for about two weeks, and then their foliage looks dreadful, except for zagreb, which looks fine after a hard cutting back. I have stopped growing them altogether. The wonderful thing is there's a zinnia bloom that looks almost like any rudbeckia or coreopsis that you want. (Before anyone is offended, please know that I love both these perennials, but they simply do not perform in the Deep South like they do further north. I grew them in Indiana with great success...)

Melampodiums are my second favorite yellow annual (the color is just a bit warm); marigolds my third favorite (I plant them for fall in place of mums which also do not do well here); Lemonbird Cosmos has to be constantly deadheaded and won't live the entire season even with attentive care (it does reseed though); lantanas take up alot of space, but in the right place are unbeatable; in my heat, petunias don't last much past June.

I am not trying to be a know it all here. It's just that I have a very long border that is yellow and blue. If I can find it, there's not a yellow foliaged or blooming plant I won't try. Therefore, I can speak to alot of varieties. And since I live in the same zone as you...

I have not tried yellow celosias or osteospermums. I was under the impression that osteospermums are cool season annuals in the south, but perhaps I am mistaken. The yellow celosias I have seen are on the gold/orange side of the spectrum and at least the red ones seed ALL OVER THE PLACE. But I do love spikes. Rhizo, is there one I'm missing?

I wouldn't rule out some yellow types of dwarf snapdragons. I have a lovely, little bushy one growing right now. Loves the sun and comes in at about 10 inches high. You don't get those nice long spikes of flowers, but it looks great.