Three things I am finding very useful for monitoring topics and clients within the realm of lovely Twitter:
Twist — Real time tracking of trends in Twitter – compare 2 trends graphically over a 7 or 30 day period. Great for showing clients how they are fairing against competitors or comparing one brand vs another for the same client. Can also embed the dynamic or static charts into your site for ongoing tracking.
Twilert — Email alerts when stuff you want to keep track of is mentioned on Twitter. Daily, weekly, monthly options available. Useful for longer term research and analysis.
Twitter search — oldie but goodie great for real time monitoring e.g. this morning for keeping on top of N97 news. Like the detail and the flow of tweets this provides.