I'll toss in a vote for Over the Rainbow, which is online at overrainbow.com
A huge selection of fabrics, at very good prices!

I actually got to visit there today! I had no idea they were within an hour's drive of me. They are not a storefront, however invited me to drop by to pick up my order. It was amazing to see the huge inventory and they are the nicest gals there.. I left with a very cut Purple Olfa cutting mat which I am gifting to my quilting buddy who loves pink and purple.

When I can't get to a shop in person, my favorite online shops are:
Over the Rainbow (I shop here most often) http://www.overrainbow.com/
Webfabrics http://www.webfabrics.net/
equilter http://www.equilter.com/
Thousands of Bolts http://www.thousandsofbolts.com/
Abbi Mays http://www.abbimays.com/
Marshall Dry Goods http://www.marshalldrygoods.com/store/

Forgot one! eBay! But you have to know what you are bidding on and figure in the shipping to know if it's a good price. I've seen some good deals on charm packs and FQ bundles. Some yardage too. I just got 4 yds of a tropical fish print by Springs industries. Very silky feeling! Price including shipping was $18.

http://quiltinaday.com
http://hancocks-paducah.com
http://connectingthreads.com
These are my favorites and just wait till you get your first catalog from hancocks of paducah with your first order.

I order patterns and kits from Connecting Threads more than other shops. There are many great online shops, sign up for the newsletters from the ones you visit and you'll find lots of bargains and sales.

Fabric.com, especially when you have a discount code
Connecting Threads (they frequently have discounts, too)
Hancocks of Paducah (their catalog is GORGEOUS!)

I just placed my first order with J Hittle, his prices are really good, plus free shipping with $100 purchase. I just ordered (and received today) the Ironslide 2000 from Joann, and wish I had waited and ordered from J Hittle, it's cheaper. He has the Supreme Free Motion Super Slider for $18.90, Machingers for $5.08, etc.

I've looked at Thousands of Bolts, but the pictures and colors are so hard to see.

I usually shop at Favorite Fabrics. http://www.favoritefabric.com They have an awesome amount of novelty fabrics. I have found some of the best patterns there! What is nice is that they have a search box, and also show other fabrics that match with each fabric. This way I don'y worry if it will match!
http://www.favoritefabric.com