The following is a brief list of popular AEG makers with brief comment about their reputation. It is by no means definative or complete as there are dozens more makers in Asia and more are finding distribution in the US every month. This is a primer and we recommend you do you're own research.
The Established Brands in Asia
These are recommended established makers who make models starting at $250 and up. While more expensive than the clones, they have been around for years and their reliablity has been proven by thousands of players worldwide. 9 out of 10 experienced players will tell you that you're better off saving your money for one of these as your first AEG.
Tokyo Marui (TM)
The GOLD STANDARD. They invented the AEG in 1992; the FAMAS F1. They feature high quality control, dependability and detail. Almost all the other makers copy TM's designs or try to make their parts compatible with TM. Designed for the Japanese domestic market, which has a maximum 1 Joule (~300fps) legal limit.
Classic Army (CA)
Based in Hong Kong, one of the highest regarded AEG makers. They feature highly detailed, high quality exernals. Their MP5's, M15's, G3's, AK's etc. come with stock with metal bodies. Their AUG and G36's come with fiber reinforced nylon bodies (like the real guns). Thier internals come pre-reinforced with high speed motors, upgraded gears, etc.
ICS (I Chih Shivan Co.)
Based in Taiwan many of their AEG's come with metal external bodies and is the only member of the Big 3 that offer their stock AEG's with +350fps springs. Some of their parts improve on TM's designs and are not TM compatible.
G&G (Guay and Guay)
Also based in Taiwan, they specialize in making excellent pre-upgraded V2 based AEG's (ie. M16, MP5) with high-quality full-metal bodies, tightbore barrles and unlike ICS, their designs are TM compatible. Be advised, their sub $200 models do not have metal bodies.
G&P
Based in Hong Kong, they specialize in making excellent pre-upgraded M-16's, M249's and M14's with high-quality full-metal bodies with full trades. Like CA, they feature 7mm gearboxes but with 350fps, M110 class springs.
STAR/ARES
Based in Hong Kong, specializes in ultra detailed replicas and some of the most expensive.
KWA
Taiwan based maker originally known for their GBB pistols and now specially upgraded AEGs. They've started with a G36C and M4 that feature 8mm bearing gearboxes with upgraded internals.
SYSTEMA
Japanese brand that specialized in high-end after market upgrade parts for Tokyo Marui AEG's. They're recently created their PTW (Personal Training Weapon) line of M4/M16's designed for the military and law-enforcement. The most expensive AEG's at over $1300/ea.
CLONES and 'Budget' AEG's
2007 was the watershed year for many mainland Chinese clone makers. Previously Clone AEG's were substandard AEG's sometimes called LPEG's. Many of these newest models appear to be almost perfect copies, though long-term reliablity issues are unknown. Here are some of the more popular clone makers:
Matrix
Matrix is Evike.com's house brand and the name it uses for AEG's and products it buys directly from the manufacturers in China. This is to give a familiar name to Chinese brands that are not well known like AIMTop, AGM, Kart, DE, etc. (see below)
JG (Jing Gong)
In 2006 the US Company Echo-1 contracted the Chinese manufacturer JG to create an upgaded copy of the TM MP5 and M4 shooting at +350fps stock. JG now sells these ugpraded AEG's to other importers under their own brand name. The newest JG M4 product line offers stock guns shooting upwards of +400fps.
ECHO-1
A Los Angeles based company started by airsoft enthusiasts. They custom contract manufacturers such as Jing Gong, CYMA, and AGM to create their line of AEG's. They were instrumental in creating US field ready stock AEG's firing +350fps.
CYMA
CYMA mostly known for their LPEG's and cheap springers, they were one of the first clone makers to make a decent versions of AK47 and MP5's, though with non-TM-compatible hop-up systems which made upgrades problematic. Their newest models are TM compatible and feature much better build quality. CYMA has recently specialized on building a range of good quality AK's models.
A&K
A&K was one of the first clone makers to come out with a copy of TM's S-Sytem M4. Featuring 300-350fps stock gearboxes, they suffered from early quality issues. More recent versions seem to be more reliable. They were one of the first to offer clones of M249 SAW's and SR25's counter-sniper rifles.
UTG
UTG is the tactical/airsoft brand of Leapers USA. They contract DE, CYMA and AGM to build their AEGs. UTG guarantees their AEG for 30 days and can easily be contacted in the US.
DE (Double-Eagle)
DE is the only clone maker of the UMP and a number of other AEG's. Their sub 300fps gearboxes make them ideal for CQB. DE makes a half-decent AK Beta though their LPEG quality G36 and SL8 should be avoided.
BOYi/DBoys/Kalash
D-Boys is the new brand name for the LPEG maker BOYI which made kids versions of airsoft guns in Asia. Like CYMA, JLS and JG they had a poor reputation intiailly due to the cheap kids guns and clones they first made. But their new D-Boys line features good copies of TM's M4. Though not as high quality as JG. BOYI also makes excellent AK's under their Kalash brand.
AGM
Contract manufacturer for Echo-1, Classic Army and others, they feature well made M14's, P90's and the H&K416 among others.
JLS
Best known for making cheap Electric Blowback Pistols (EBB). JLS recently came out with the first clone SCAR. Not an exact copy of the TM/CA SCAR and is not as ruggedly built but much cheaper, though its long term reliability is unknown.
WELL
Well was an maker of LPEGs and cheap springers. They featured cheap externals and unreliable gearboxes. They have released improved models of their M4 variants but their quality and compatibility falls below that of JG, DBoys and others. Apart from their MP7, it's not a brand I recommend.
BE (Both Elephant)
A maker of LPEGs that has since upgraded to metal gearbox clones, many of their AEG's still suffer from thin plastic bodies and TM incompatibility issues. Avoid.
AIM "TOP"
Taiwanese company with manufacturing in mainland China. The first affordable (under $120) clone of the Tokyo Marui M4 but with quality control issues. Eclipsed by JG's clones but have come back with some pre-upgraded M4 models that are very high powered.
Crossman
A big name brand in paintball which has attempted to cash in on airsoft. Rebranded various low end products under their Crossman brand name including WinGun and HFC for their gas and spring pistols and DE for their AEG's.
VFC
(Vega Force Company): Specializes in some of the best and most realistic looking AK variants money can buy. Also has one of the most realistic SCARs on the market with ultra-realistic take-down. Most have metal bodies.
SRC/TSD:
TSD rebrands SRC I believe, they make very high quality G36 variants, better, although more expensive than JG and Echo 1. They also make reasonably well made M4/M16's with full metal bodies and released one of the best HK 416's with metal bodies and 8mm gearboxes.
King Arms:
Mostly known for their high quality aftermarket externals they specialize in very unique "TROY" M4 variants. Also one of the only airsoft companies to manufacture a FAL.
Real Sword
Real sword is well known for making über realistic AEG's. There is a downside. Their AK is exactly the same dimensions as a real AK but this meant that their gearbox is a slightly smaller than a TM gearbox so many parts are not compatible. But this does mean it's compatible with some real steel furniture and accessories (in other AK AE's you'd have to do a lot of modification work.)