Where Have I Seen…
I have, for a long time, dreamed of a Firefox extension that would do this. Now, finally, empowered by Greasemonkey, I wrote one myself. (Technically, a Greasemonkey user script is not a Firefox extension, but it can be easily converted into one, and a Greasemonkey script is the Right Thing anyway.)
What it does: on any IMDB cast listing, show for each actor what other movies you have seen him/her in. Here’s a screenshot:
It’s currently named “WHIS”, can you think of a better name? :)
Update: It’s now a full-fledged Greasemonkey script, and is up on userscripts.org.