"Key" seems to mean quite a few different things. Good catch, <s>it would involve a lot of manual retagging though</s> doesn't matter anymore though :P

Let's see:

### Keyboard keys

Both the physical ones on the keyboard and the conceptual ones like hotkeys, key-combos, and keystrokes that perform functions. Mostly in combination with [tag:keyboard], [tag:keyboard-shortcut], [tag:keyboard-layout], [tag:hotkeys].

### Registry key

The ones used in Windows Registry. Mostly in combination with [tag:registry].

### Cryptographic keys

Encryption keys as in PGP. Mostly in combination with [tag:php] or [tag:cryptography].

SSH keys generated by `ssh-keygen` tool. Mostly in combination with [tag:ssh].

### Database keys

Keys as used in databases. Mostly in combination with [tag:odbc], [tag:access], etc.

### Serial numbers

CD keys, product keys, serial numbers, et. la. Mostly in combination with [tag:registration] or [tag:activation].