Here are some tools you can use with Twitter to find out about trends.
Flaptor Search
Search tool that can be installed on your browser.
Favotter
Find out the most favored (saved to favorites) tweets
GoodBadMe
Keep track of the good and bad with the #GoodBadMe hashtag
Tweetmeme
find out the most tweeted blogs, images & phrases
Tweet Scan
Search keywords on Twitter
Twistori
Find out what twitter’s love, hate, think, believe, feel & wish
Twitbuzz
Find out the latest links and conversations on Twitter
Twitscoop
A search tool that tracks trends, key words & topics
Twitstat
real time twitter analytics from your mobile device
Twitterment
Search Twitter users and topics
Twitter’s Search
Search for the key words, names or the latest trends on Twitter
Twitterverse
Find out what Twitter is doing
Twitturly
Find out the most talked about 100 stories on Twitter