Are there any important, extraordinary, or accomplished Hufflepuffs?

Cedric Diggory was a prefect, and captained the Hufflepuff Quidditch team, playing as Seeker.

He was also selected as the Hogwarts Triwizard Champion, and, although overshadowed in the press by the entry of Harry Potter, Cedric enjoyed almost overwhelming support from the student body, and, after the first two tasks, tied with Harry for first place.

So, that is surely an accomplishment for a Hufflepuff!

Adding to freaknsick's answer, there were some other Hufflepuffs those surely earned a good name for their House:

1. Pomona Sprout: Head of the Hufflepuff house and a Herbology teacher.

2. Nymphadora Tonks: She was an accomplished Auror, and also a member of the second Order of the Phoenix.

   Then, before Harry Potter's age, there were:

3. Bridget Wenlock: He was a famous thirteenth-century Arithmancer, and the first to establish the magical properies of the number seven.

4. Newton Scamander: He was a famed Magizoologist and author of Fantastic Beasts and Where to Find Them.

   and lastly, there was:

5. Helga Hufflepuff herself, who was one of the four founders of Hogwarts School of Witchcraft and Wizardry. I assume she accepted students on the basis of traits that defined her personality, which mean, had she been sorted into some house, she would definitely have been a Hufflepuff.

From The Harry Potter Wiki:

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Also, from Pottermore Hufflepuff welcome:

Hufflepuff is certainly the least boastful house, but we’ve produced just as many brilliant witches and wizards as any other. Want proof? Look up Grogan Stump, one of the most popular Ministers for Magic of all time. He was a Hufflepuff – as were the successful Ministers Artemesia Lufkin and Dugald McPhail. Then there’s the world authority on magical creatures, Newt Scamander; Bridget Wenlock, the famous thirteenth-century Arithmancer who first discovered the magical properties of the number seven, and Hengist of Woodcroft, who founded the all-wizarding village of Hogsmeade, which lies very near Hogwarts School. Hufflepuffs all.

So, as you can see, we’ve produced more than our fair share of powerful, brilliant and daring witches and wizards, but, just because we don’t shout about it, we don’t get the credit we deserve. Ravenclaws, in particular, assume that any outstanding achiever must have come from their house. I got into big trouble during my third year for duelling a Ravenclaw prefect who insisted that Bridget Wenlock had come from his house, not mine. I should have got a week of detentions, but Professor Sprout let me off with a warning and a box of coconut ice.

However, it’s true that Hufflepuff is a bit lacking in one area. We’ve produced the fewest Dark wizards of any house in this school.