1. Weapon Mods: this mod adds the ability to customize your weapons. Want to add a silencer, a laser sight and a scope to your favorite assault rifle? Of course you do!
Sometimes modified weapons are found, but usually WMK's (weapon mod kits) of a particular type are found as treasure, leaving the modding of the weapon up to you.
This only possible downside to this mod is that it only covers small guns. If you love big guns or energy weapons, this mod won't add much to your game, though you can just sell any WMK's you come across.
This mod also takes an already important skill, Repair, and makes it even more important, since your Repair skill will affect the condition of any weapon you mod (modding it makes its condition go down).
2. Repair Rethought: This mod tweaks the way equipment repair works, making it more flexible. For example, you can cannibalize an assault rifle to repair a Chinese assault rifle, or a 10mm pistol to repair a 10mm SMG and so forth.
This mod also allows some junk items to be used for repair. For example, scrap metal or Abraxo cleaner can both be used to repair weapons now, glue can be used to repair cloth armor (like your armored Vaultsuit), leather belts can repair leather armor, and so on.
This mod also makes repairing homemade weapons (like the Railway Rifle) much easier. Instead of making two Railway Rifles so you can repair one with the other, this mod allows any of the parts used to make an item usable for repair. So if you have a Dart Gun, you can use toy cars (one of the things you used to make the Dart Gun to begin with) to repair it.
3. Better Living Through Chems: This mod adds a ton of new drugs to Fallout 3, including many real world drugs, like morphine and heroin. It also tweaks the way addiction works, and allows you to use hypos as weapons. Find enough methamphetamine to kill a supermutant? Stab him in the leg with a hypo and do just that!
4. Wearable backpack: Are you a packrat who is always wishing you could carry more? This mod adds a backpack as an item you can buy for 500 caps (give or take, based on your Barter skill) that increases your carry capacity by 50 lbs.
5. Ammo crate: This mod adds a new item you can buy for your house in Megaton or Tenpenny Tower that basically gives you access to an ammo delivery service. Once you pay your 700 caps for the ammo crate (again give or take based on barter) you can then pay to have a specific amount of ammo delivered instantly to your house, once per day.
Not only does this ensure you have plenty of 5.56mm rounds for that tricked out Chinese Assault Rifle you modded with an extended clip, a laser sight and a silencer, you also get the ammo at a slightly cheaper rate than if you bought it at the store.
Of course, you're not really coming out ahead, since you paid 700 or so caps for the crate to begin with. Mostly this item is just a convenience. It stops you from having to make a run all the way to Rivet City just to see if Flak has flamer fuel in stock. Just deduct the caps and get what you need from your ammo crate.