Many sites offer some statistics like \"The hottest topics in the last 24h\". For example, Topix.com shows this in its section \"News Trends\". There, you can see the topics
I had worked on a project, where my aim was finding Trending Topics from Live Twitter Stream and also doing sentimental analysis on the trending topics (finding if Trending Topic positively/negatively talked about). I've used Storm for handling twitter stream.
I've published my report as a blog: http://sayrohan.blogspot.com/2013/06/finding-trending-topics-and-trending.html
I've used Total Count and Z-Score for the ranking.
The approach that I've used is bit generic, and in the discussion section, I've mentioned that how we can extend the system for non-Twitter Application.
Hope the information helps.