Search the Web!
Before I begin with the actual web sites that I use, I need topoint out that there are many good web search engines that you should be using to find the information you need. The best I have found is Google. I openly admit that when someone asks me for advice or information, I open my browser, search google for the topic, and get all of the info I need for an intellegent response.

Online Fish Stores:
LiveAquaria is an awesome site for mail-order animals. The site is very informational regarding the animals, and the high quality pictures make identification easy.

Drs. Foster and Smith is the dry goods store associated with LiveAquaria. They have everything for every kind of pet.

Premium Aquatics is where I purchased almost all of my accessories and a lot of my livestock. They have a great staff, a very large selection, good inventory and good prices.

Marine Depot is another good store - especially for lighting supplies. I always shop both Premium Aquatics and these guys before I place any orders.

That Fish Place is in Lancaster PA and every once in a while I drive out there (about a two hour drive). They have a great staff and a really nice store, but their web-site is not so easy to navigate. As well, they have been short-handed in the fish room lately and the animals have not been taken care of as well as I like to see. For anyone who lives in the area, it's a worthwhile trip - they have an indoor ray pool and allow children to pet the rays. They also offer some neat programs for children. On a final note - when you visit the store there is a very nice older gentleman greeting you at the door (named Bob). You would not beleive how much he looks like the cartoon on the homepage of their web site.

Live Rock and Sand (and stuff):
Live Rock, Live Sand, sand bed primers and detrivour/algaevour kits can be purchased from most of the above live aquaria dealers. However, here are a few people who speciallize in these things:

Tampa Bay Saltwater is by all means the best place that I have heard of for Live Rock. I have not used them personally, but I have seen the testimony of many happy reefers on the newsgroups. Most of the time they include comments like "OH MY GOD!". These guys actually have a Live Rock farm off of the coast of Florida. They plant dead rock,let it become inhabited with all sorts of life and harvest it. It is a very ecologically responsible way to supply the aquarium hobby with Live Rock.

Inland Aquatics is one of the places I use for sand bed fauna primers, coral polyps and other good stuff to get your micro-critter population up. You can also find information here about the use of refugiums to complete the biocycle in your system.

Indo-Pacific Sea Farms is another great critter supplier. They have a really broad line of things designed to get the fauna system started in a Deep Sand Bed (DSB) or refugium system.

Everyone eventually needs some things that you just can't find (or can't afford) at the local fish store or online aquarium supply site. For those times, I recommend McMaster-Carr Industrial Supply. They handle EVERYTHING - plumbing, hardware, plastic, glass, electrical components, etc...

Information:
There are so many places to go for information on the Internet that I can't possibly begin to list them. However, the places that have been helpfull to me are: http://saltaquarium.about.com, and the newsgroups rec.aquaria.marine.misc and rec.aquaria.marine.reef. Again, search Google for information on any subject you can think of.