IconVault's Keycode Tester shows the full KeyboardEvent details of any key you press. Click the capture area, press a key, and you instantly see its key, code, keyCode, which, location (general, left, right or numpad) and any modifiers (Ctrl, Shift, Alt, Meta), which is exactly what you need when debugging keyboard shortcuts or remapping keys.

Every press is logged to a history table (the last 50 events) so you can compare keys and spot inconsistencies across browsers and layouts. Everything runs fully in your browser with nothing uploaded, it is free to try (heavy repeated use may ask for Pro), and there is no signup.