Here's a user made list of online suppliers.

A lot of these sources are unknown to me. If you've dealt with them please include when (ish) and a rating on
your experience. 0-5 scale. 0=bad, 5=excellent. Try to include a link to the page.
We'll put this into 5 categories. The categories are simple. Firearms, Ammo, Blades, Gear, and Other.
I understand most of these companies sell more than just one type of thing. Try and put them under the most suited area. If the supplier has more, mention it in somewhere in the description.

Firearms
Bond Arms: Produce high-calibers derringers. Home offices in Texas, USA.
(Add any experiences here.)
CanadaAmmo: Decent price, great quality. Has the non-Americanized (which is really stupid looking) version of the TAR-21. Unfortunately, no STAR with heavier barrel and improved optics, although you can buy the ACOG sights from Trijicon instead. (After a short look around their site, it would appear that they do sell ammo as their name would suggest, but they tend to be firearm heavy, earning them a spot here.)
CDNN: They have a little bit of everything but most of their stock is firearms or firearms related.
I give this site a 4 out of 5. I bought 2 mags to a handgun (fall of 2010) that are hard to find from these guys. What I liked was that they actually have non factory, non specific things linked to guns that will work in the gun it's linked to. I got two magazines made by EAA for a magnum research gun at a good price that have worked flawlessly. Also, I was pleasantly surprised to find a free mag pouch with my magazines. I will be doing business with them again - Jahadaz
Cheaper Than Dirt: Like guns? Like shooting? Check these guys out. Gun accessories (including upper receivers for the AR-15 Series), ammo, cleaning kits, if it has anything to do with guns, even many gunsmithing tools, you name it, they got it. (These guys have everything. Literally everything. I've never bought from them but I know several members that do regularly and love them)
Keep Shooting: AK derivatives and a ton of surplus gear, like gas masks, body armor for super cheap, smoke grenades - some crazy stuff like urine test dupers too. (I thought the prices were a bit high overall but did see a few steals here and there)
Gunbroker: Basically Ebay for guns. (Very well known site)
Ammo
AIM Surplus: These guys have a TON of ammo. Almost every type you can imagine for competitive prices. They also have some really good prices on surplus firearms.
Ammunition To Go: As the name suggests, this site is based almost entirely around ammunition. They carry most kinds of ammunition, and also carry "novelty" ammunition like starburst rounds. They also carry a number of accessories and knives.
(Order from them all the time. Great prices most of the time, and prompt service and shipping. Carry lots of hard-to-find ammunition)
LTD Ammo :
Midway USA: This is another site that has a ton of different things but is most popular for it's ammo and reloading supplies. Several members deal with them regularly.
Palmetto State Armory:
SGA AMMO
Reloading
Widener's Reloading Supplies:
Midway USA: This is another site that has a ton of different things but is most popular for it's ammo and reloading supplies. Several members deal with them regularly.Lee Factory Sales:
Sells Lee reloading (great beginner reloading stuff) supplies at less than any other site.
Starline Brass: Sells great brass
Powder Valley: Sells almost everything you need for reloading from bullets to primers to brass to the obvious powder.
Friendswood Bullet Co: Sells really well priced lead bullets.
Once-Fired Brass:Sells once fired brass. (duh)
Blades
BudK: Knives and melee weapons; One would be best-profited to obtain working knives and blunt weapons from them. (Looks like they also sell random surplus, I noticed masks, flashlights, and toys also on their main page)
Roadside Imports: Just found on the 'Net, they have a wide assortment of "spring-assisted" and automatic knives, some well known brand names. Also, they do have some standard knives and tools, but the emphasis is on spring blade type knife. (Might have been sold or renamed from when this was added because the link goes to Assisted Knife)
Swords 24: European supplier of knives, swords, and similar items, based in Poland
Gear & All Around Suppliers
That Don't Have Firearms
Blackhawk: Military, Law Enforcement, Corporate Security, and Outdoor Gear. (I've owned several blackhawk products over the years and I have to say, they are well worth the cost. If you can afford it. Never dealt with the site though) (Made in Vietnam, quality is not as great as some others and may not stand up well to regular rough use)
London Bridge Trading Co.: Expensive but very high quality tactical gear, their selection is insane and many of their products are in use by the US military.
Tactical Tailor: Yet again, expensive but of the highest quality. Primary market seems to be military.
Galls: Uniforms and duty gear such as gun belts, body armor, ect. (I saw a large amount of 5.11 gear on this site)
LA Police Gear: Riot gear (Looks like another cop site to me)
Major Surplus and Survival: Military surplus and survival gear. (Looks like a fairly decent all around surplus site)
OpsGear: Arguably one of the best sites for any type of gear. Created after the 9/11 attacks to service Military, Law Enforcement, and Civilian needs alike. They also offer urban combat training for Military and Law Enforcement individuals. For Civilians they also hold Military courses such as escaping captivity and various types of self-defense.
Pernith Survival Equipment: General survival equipment supplier. Based in England.
Qaurtermaster Uniforms: Cheaper duty gear. (Looks like another cop site to me)
Surplus City: Bills themselves as the largest surplus store on the west coast.
Uncle Mike's: Duty gear for cheap, cheap cheap. Known for plain kydex and nylon stuff. (I noticed a ton of holsters and range gear on this site. I know it's popular among Law Enforcement)
Other
eFood: Bulk food. (As advertised, food storage only. I would like to know if anybody has tried these guys before because their year long supplies are pretty cheap)
The Epicenter: General survival supplies, plus solar power systems and other items for long term events.(While they do have some gear most of their stock looks to be emergency style food. MRE's, food storage, ect.)
OTHERPOWER: Alternate energy source.
Surefire: They make some of the most retardedly bright and durable lights on the market. Very reliable.
Trijicon: Arguably some of the best optics around, many with nighttime illumination that does not require batteries, and noted for reliablility. Reflex sights DO increase targeting speed and acquisition, enhancing CQC shooting vastly. The ACOG sight is immensely popular with US military forces.
Disclaimer - the ZSDW is not responsible for any problems it's users may encounter with any of the sites on this list. This is a user made list made with the intention of users adding sites they have personally dealt with and feel the need to share with others.