Check out this free twitter trend archive. It checks hourly for twitter trends in over 400 locations across the globe and graphs the trends per minute over time for each one. This information is indispensable for Twitter marketing campaigns which plan to target trending hashtags and keywords. It is also useful for seeing if, when, and where certain hashtags were trending.
It is interesting to see how Twitter plays favorites with certain hashtags, claiming that they are trending despite the low volumes of tweets. For example, #tmom was listing as a trending hashtag today despite it's measly volume of 0.1 tweets per minute. Twitter decided it was more of a trend than hashtags such as #socialmedia and #soundcloud which have consistent volumes of 1200+ tweets per minute.