Art Fibers is by far my favorite yarn store. If you live in or plan to visit the Bay Area, and you love knitting or crocheting, this is a must on your list of places to visit.
Art Fiber's yarns are sold exclusively by them. All of their yarns are made for their store, and their stock does change over time (as they add new yarns and discontinue others). If you see something you love, buy it -- otherwise, you may never see it again.
They carry yarns ranging from wools to cottons to eyelash yarns to ribbon yarns, and more. Their yarns are very high quality. However, you will pay for that quality. These are not inexpensive yarns. However, prices are reasonable. Expect to pay anywhere from $8-$15 (there are some outside that range) for a 50g ball of yarn.
The store offers sample balls of all of their yarns (except for a small selection of very special, limited quantity ribbon and hand-spun yarns). There is a circle of comfortable chairs, and a supply of needles that you can use to work up a swatch of their yarns. This is great for seeing how it might look in your pattern, or mixed with other of their yarns.
Each of their yarns has sample swatches made, and displayed so that you can evaluate the feel, drape, thickness, etc. Sometimes they'll have swatches of combined yarns, to help inspire you. And there are also a number of rotating finished projects, made with their yarns, in the store, to further help give you ideas.
If you don't already have a project in mind, the store has knitting pattern software. If you buy the yarn for the project through them, they will generate a free pattern, based on your measurements and specifications. They also have patterns for sale.
And is if all of that wasn't enough, the staff at Art Fibers is amazing. They are very knowledgeable, and are happy to help you plan your project. Sometimes I'll go in with an idea, and they'll offer advice on which yarns to try, or techniques to use. And when I come back next time, they'll remember and ask how my project is going (they'll even remember what the project was).
For people outside the area, Art Fibers also has a website, and will send samples of their yarns for you to try. And, you can also order most of their yarns through their website.