I like Yahoo Finance Stock Chart, and the way you can filter by a date range:
I have a database of news headlines, and I'd like to replace our date filter, especially for the tag cloud, with some sort of dynamic range widget. I think most of our users are going to want to see headlines and tags from the most recent X days, so I could just use a regular slider, but the idea of being able to slide both the timeline length (3 months) and range (Sept '04 - Nov '04') seems pretty cool. Specifically, with the cloud tag you could visually represent which tags were popular during one particular time. Like "trending".
Finally, adding volume (headline count) like the stock chart volume could be interesting as well.