Humble Steam Check

Stop buying games you already own. Humble Steam Check automatically shows which games in any Humble Bundle are already in your Steam library. Green checkmarks for games you own, red X's for games you don't. Works on bundle pages, store pages, and subscription trays. Just enter your Steam API key and Steam ID in the popup, and badges appear next to every game title. No data collection, no external servers — everything runs locally in your browser.