I have a bunch of items that I would like to add to my Fallout world. Extra weapon types for raiders and such.
How can I add them to the things that raiders and other wasteland denizens use?
Is there a list or something that I can edit in the GECK?

Also, how are the items that appear in shops controlled? How can I add/remove items to the list of things that are sold in each location?

Any help with this most appreciated

In short :
Leveled Lists

Slightly more verbose:
Open the GECK and go to Actors>NPC. For an example type AAFTemplate into the filter. AAF stands for African American Female. These are templates because all races and genders will copy whatever settings are found in these templates. You'll note that the naming conventuon is NPC Type, Level, Equipment. Scroll down and find the Raider section and select Raider1GunAAFTemplate. You can cruse though all the different tabs, they all do really cool stuff, but for now lets look at the Inventory tab. You should see 4 leveled lists, Armor, Gun, Backup weapon and Loot. Open up Raider1WeaponGun and Raider1WeaponGunBackup25. An important note here is the Chance none, there is zero chance that a gun raider will spawn without a gun, but a 75% chance they will spawn without a backup weapon. Also note that the Use All box is not checked on either list. This means that when the raider is spawned onle one item from each list will be used. Now from the gun list open the WithAmmo10mmPistolRaider, here the Use All is checked so the raider will spawn with one pistol, one clip and have a 75% chance for each of 9 additional rounds. Here you will see that the pistol, as was the knife and knuckles in the backup weapon list, have the prefix Cond-, this controls what condition the weapons are in. Open the pistol and you see that a level 1 raider can only have a 10mm pistol with 25%, 30% or 35% health.

Now if you want to add your own weapons you need to go to Items>Leveled Items and make your own WithAmmo list. then go back to your raider template and right click to add items to the existing leveled list. For compatability reasons a lot of modders use the AddItemToLeveledList command and do it through a script.

I hope this helps, let me know if you have any futher questions about leveled lists.

Oh you also asked about vedor lists, goto Items>Leveled items and filter for VendorChest. These are the leveled list for the vendors, right click > Use Info will show you which vendor containers hit on that specific list. If you want items added to multiple vendors then add them to the leveled list, if you pnly want them added to specific vendors then create you own leveled list and add it to that vendors container.

Thanks for the very detailed and clear explanation Steve!
I will give that a shot and get back to you if I have any questions.

On a related note, I've been using your Savage wasteland/Unfound stuff with new character and liking it alot.
On the other hand, I am finding that once I get some decent firearms I have a huge advantage over raiders, since I have also modded the balistics to give much more realistic accuracy and damage.
It's basically the old joke about 'bringing a knife to a gunfight'!
Without some sort of ranged ability, raiders really don't have much of a chance against a player who makes planned attacks from a distance.
So I am looking at adding some crude pipe rifles and maybe a crossbow, and bolstering their danger factor when they do get close by swapping out lame weapons like pool cues, tire irons and switchblades for Axes, machetes and spears.

Also, Ihave noted some anomalies.
I am finding a fair number of raiders who don't seem to have any weapons at all, while there are some others (e.g. the raiders on the overpass near sprinvale) who were half unaramed and the others had a sniper rifle and 3 flamethrowers!
A bit of surprise to say the least, especially since I was only 2nd level at the time.

Overall great, but I think I may have a go at tweaking the mix to gel with my other changes.

http://www.fallout3nexus.com/downloads/file.php?id=12307

Has a pipe rifle and spears and other crude weapons. It's still in beta, more weapons and fixes for bugs on the way.