Nat recently implemented tag-typing for F-Spot, our new photo management software. For those of us with large picture collections this is probably one of the best interfaces for tagging.
Send copious amounts of email to Nat until he does a screencast of this new feature. You have to see it to appreciate it.
On the hacking side of things, the beauty of this patch is that it was coded in two afternoons after long busy work days for Nat.