My system has evolved into multiple Word documents. Separated by model type (T2x, X2x, Z6x, A2x, A3x, etc.) into .doc files, they have been posted to my Dropbox account so I have now have access to them as long as I have one of my laptops with me. Thought about putting them into an Access database but haven't gotten around to it and also because I don't have the full Office suite loaded into every one of my computers. I've also thought about using my copy of ClarisWorks for Windows (how's that for an old, obscure database program?), but inertia wins out again.
Data about the systems include who it was bought from and when, the model type, serial number, motherboard serial number, BIOS/EC version, CPU, memory configuration, UUID, MAC address, hard drive info (manufacturer, size, model number, OS installed), screen size and resolution, battery info (serial number, full charge capacity and charge cycles), how much it originally cost, and in most cases, what I've additionally spent on it. Included are notes about passing my usual sequence of diagnostics, any known issues, damage or missing pieces, as well as who I sold it to and when.
Inventory and location of spare parts such as Ultrabay HD adapters, Ultrabay optical drives, power supplies, docking stations/port replicators, external optical drives, memory modules, hard drives, floppy drives, batteries, motherboards, lids, LCDs, cables, screws, caddies and what not, is kept in my head

. Most of this stuff that is commonly used is within eyesight anyway, much to the consternation of my wife when she looks into my home office.