Hi all,
I run BrickMissing (www.brickmissing.com), a free parts checker I built after losing one piece of a set and realising I had no idea whether that part even existed anywhere else. It covers 17,000+ official sets. Every set page shows the full inventory with official Pick a Brick prices per part, plus what I call a repairability score: the share of a set's part+colour combos that appear in 3 or fewer sets ever, so you know before you buy secondhand how risky missing pieces are.
Building the score meant going through 1.13 million inventory records, and some of what came out surprised me. Half of all part+colour combos LEGO ever made exist in exactly one set. And sets from 2000-2004 are the hardest to repair ever made, you can literally see the pre-2003 mould explosion in the data. The full study is on the site if you like that sort of thing.
You can also track which parts you own and see which sets are buildable with them. Browsing needs no account and the whole thing is free, this is a hobby project that got out of hand.
Genuinely keen on feedback from people here: what would make this useful for how you actually manage your collections? The data layer can answer more questions than I've built screens for, so suggestions are welcome.
Robert
