There will be moments where you'll have something that is seemingly useless crap sitting in your bag, encumbering you and generally being seen as nothing more than mere merchant trader trash to sell off. Well, whether or not you know it, that seemingly useless lawnmower blade you just sold is part of a recipe for a custom weapon, and there's no telling when you'll find that item again.
To avoid this kind of situation, I've decided to list a few of the recipes I've discovered so far, and to put them up here. Keep in mind, you cannot actually build these weapons without two very important things. First, you need the actual schematic, which you can either earn as quest rewards, buy from a merchant, or if you're really lucky, find while sifting through a locked something or other. Second, you'll need a workbench, and good luck finding one. I think there's one in the town near the vault, but I don't remember anymore, since I use the one in the apartment I own.
So long as you have these two things, you can build any which one of the custom weapons you have schematics for, provided you have the proper materials. This is by far not a complete list, as it's only of what I've found so far.
Nuka Grenade - Tin Can Turpentine Abraxo Cleaner 1 x Nuka Cola Quantum
A potent cocktail of carbonation and detonation, the Nuka Grenade makes plasma look like a bubble bath. Makes one grenade.
Also keep in mind, if you happen across a second schematic after you've already learned it, you'll learn a version 2.0 of that weapon. It may confer certain bonuses, such as extra damage or, in the case of the lunchbox mine, making two for the price of one.