In order of usefulness:
Sling - being able to drop your gun so you can use your hands and have it still across your chest is so useful in more circumstances than I can even remember, but most obviously climbing ladders, through windows, or crawling, but also something as simple as explaining where the enemy are to other people; plus having part of the weight taken by a sling helps - even if you are strong, it's the reps that do you in, so holding your gun up all day takes it out of your biceps...
Sling Mount - your AEG's hang for comfort and ease of use can be greatly improved by where the sling attaches and what shape the mount is.
Vertical Foregrip - holding horizontal handguards requires your wrist to be rotated almost as far as it goes, which can cause an ache after a whole day, but a vert grip allows a much more comfortable wrist position.
Angled Foregrip (AFG) &/or Gas Peddle / Hand Stop / Magwell Grip - some people find these better than a vert grip.
Grip-pod - combined vert grip & bipod - can be useful, especially if defending a trench or other fixed position where you are not going to move far until you get hit, but on the sort of gun which you wouldn't ordinarily put a bipod on. Some of the tube type which have 2 legs spring out below have really crappy plastic legs which snap unless you use them very carefully.
Red Dot Sight - as Ed said, allows faster target acquisition. You need to get one which has a fairly bright dot to be useful outdoors, so the type which have variable brightness settings are probably best, unless you can get the same type someone tells you is bright enough.
Torch - you don't need to spend fortunes / Ultrafire is a cheap Chinese brand but the Cree XML T6 LED they use is the same as used in more expensive brands. The C8 model is a pretty good all rounder, but the type with a zoomable bezel is probably better as a weapon mounted torch, even though it is not quite as bright as the C8, due to having less reflector than the C8.
Rail Covers - can stop sharp edges wearing away at your clothing/equipment, but you should not be holding your AEG by a rail anyway, so it shouldn't matter to your hands - and you should also have gloves on...
Lasers - if your local sight allows green ones then they can be used outside and can be useful to show someone else where an enemy is hiding, but they're not really much use as a target designator to use in conjunction with your own sight. Red ones are not bright enough in daylight outdoors and indoors you are unlikely to need to paint a target location for somebody else.
Noob Tube - AKA nade launcher - funny to burst into a room and let go with 108 BB's in 1 pop, but doesn't achieve anything full auto wouldn't.
even a cheapo belt with a pouch is 100 times easier than filling pockets with bulky mags and losing them etc...
h34r: he used his passport) and away you go I think next day delivery, can't remember though really good and very nice chaps, not a fan of their 1 line descriptions but don't let that put you off 
