Is there a tool to find out which key phrase has been tweeted the most?

[Sparkstone]

As the title suggests ^ ^

[NiallSmith]

There are a few different tools I've found helpful to find out what's trending on Twitter, namely:

http://www.twee.co/ - what's trending now on Twitter

http://www.topsy.com - more trend information

Another new one I've found helpful for trends throughout the USA and world is http://trendsmap.com/