Any website that shows and recommends popular Twitter hashtags on a particular topic?

WhatTheTrend.com gives you insights on the top topics and hashtags that are trending worldwide.
You can even narrow your selection down to particular countries and cities.
Twubs.com is another site that lets Twitter users form groups around popular hashtags.
Since the information is user-generated, you can see what hashtags are popular under categories such as books, conferences, movies, etc.