The Harvest Sprites
They live in the shack by the church. Chef, Timid, Hoggy, Staid, Bold, Aqua and Nappy. Anyway, they help you out around the farm. Each Sprite has a heart meter, just like animals, and this counts towards how willing they are to complete tasks you give them. Just give them grape juice or flour, and they'll start loving you real quick.
They can be used to help out around the farm by either watering plants, harvesting crops, or tending to the animals. The more they do any given job, the more efficient they become at performing it. You can hire them for either an entire day, 3 days, or a week. it's your option, how long you want to hire them. Just know that after their days of servitude are up, you need to wait at least a day before they can work for you again.
Here's my view on the jobs they do:
Watering plants: This is useful near the beginning, before you get the Silver Watering Can. They can water that last plant inside the 9x9 square of crops, and just plain make it all go faster. Hire these guys by the week is my advice.
Harvesting crops: Not quite as useful as you think. Sure, it speeds up time, and can harvest that one remaining plant inside the square, but too often, I found days where they were just lounging about, which I think brings their heart levels down. (More on that later.) I never hire these guys for more than a day at a time.
Tending the animals: Don't bother with this one. They feed the animals, and take any products they have. They don't run them through machines to get a better profit, the don't rub down animals, and you still need to talk to them every day. Just don't bother.
They can't be hired in the spring, since that's apparently when they have tea parties, or whatever. If you get all their hearts to 7 or above, go to their shack on Sundays around 3:00 p.m. and bring them gifts (Actual gifts, use the wrapping paper at the store). They should invite you to the Party, and afterwards, they give you Relaxation Tea leaves, which can be used to make Relaxation tea. Yay!
And that's all you need to know about Harvest Sprites.